Add `recursive` argument to text:each() and text:map(), plus update docs

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+# Text Pattern Matching
+
+As an alternative to full regular expressions, Tomo provides a limited string
+matching pattern syntax that is intended to solve 80% of use cases in under 1%
+of the code size (PCRE's codebase is roughly 150k lines of code, and Tomo's
+pattern matching code is a bit under 1k lines of code). Tomo's pattern matching
+syntax is highly readable and works well for matching literal text without
+getting [leaning toothpick syndrome](https://en.wikipedia.org/wiki/Leaning_toothpick_syndrome).
+
+For more advanced use cases, consider linking against a C library for regular
+expressions or pattern matching.
+
+`Pattern` is a [domain-specific language](docs/langs.md), in other words, it's
+like a `Text`, but it has a distinct type. As a convenience, you can use
+`$/.../` to write pattern literals instead of using the general-purpose DSL
+syntax of `$Pattern"..."`.
+
+Patterns are used in a small, but very powerful API that handles many text
+functions that would normally be handled by a more extensive API:
+
+```
+Text.has(pattern:Pattern -> Bool)
+Text.each(pattern:Pattern, fn:func(m:Match), recursive=yes -> Text)
+Text.find(pattern:Pattern, start=1 -> Match?)
+Text.find_all(pattern:Pattern -> [Match])
+Text.matches(pattern:Pattern -> [Text]?)
+Text.map(pattern:Pattern, fn:func(m:Match -> Text), recursive=yes -> Text)
+Text.replace(pattern:Pattern, replacement:Text, placeholder:Pattern=$//, recursive=yes -> [Text])
+Text.replace_all(replacements:{Pattern,Text}, placeholder:Pattern=$//, recursive=yes -> [Text])
+Text.split(pattern:Pattern -> [Text])
+Text.trim(pattern=$/{whitespace}/, trim_left=yes, trim_right=yes -> [Text])
+```
+
+## Matches
+
+Pattern matching functions work with a type called `Match` that has three fields:
+
+- `text`: The full text of the match.
+- `index`: The index in the text where the match was found.
+- `captures`: An array containing the matching text of each non-literal pattern group.
+
+See [Text Functions](text.md#Text-Functions) for the full API documentation.
+
+## Syntax
+
+Patterns have three types of syntax:
+
+- `{` followed by an optional count (`n`, `n-m`, or `n+`), followed by an
+  optional `!` to negate the pattern, followed by an optional pattern name or
+  Unicode character name, followed by a required `}`.
+
+- Any matching pair of quotes or parentheses or braces with a `?` in the middle
+  (e.g. `"?"` or `(?)`).
+
+- Any other character is treated as a literal to be matched exactly.
+
+## Named Patterns
+
+Named patterns match certain pre-defined patterns that are commonly useful. To
+use a named pattern, use the syntax `{name}`. Names are case-insensitive and
+mostly ignore spaces, underscores, and dashes.
+
+- `..` - Any character (note that a single `.` would mean the literal period
+  character).
+- `digit` - A unicode digit
+- `email` - an email address
+- `emoji` - an emoji
+- `end` - the very end of the text
+- `id` - A unicode identifier
+- `int` - One or more digits with an optional `-` (minus sign) in front
+- `ip` - an IP address (IPv4 or IPv6)
+- `ipv4` - an IPv4 address
+- `ipv6` - an IPv6 address
+- `nl`/`newline`/`crlf` - A line break (either `\r\n` or `\n`)
+- `num` - One or more digits with an optional `-` (minus sign) in front and an optional `.` and more digits after
+- `start` - the very start of the text
+- `uri` - a URI
+- `url` - a URL (URI that specifically starts with `http://`, `https://`, `ws://`, `wss://`, or `ftp://`)
+- `word` - A unicode identifier (same as `id`)
+
+For non-alphabetic characters, any single character is treated as matching
+exactly that character. For example, `{1{}` matches exactly one `{`
+character. Or, `{1.}` matches exactly one `.` character.
+
+Patterns can also use any Unicode property name. Some helpful ones are:
+
+- `hex` - Hexidecimal digits
+- `lower` - Lowercase letters
+- `space` - The space character
+- `upper` - Uppercase letters
+- `whitespace` - Whitespace characters
+
+Patterns may also use exact Unicode codepoint names. For example: `{1 latin
+small letter A}` matches `a`.
+
+## Negating Patterns
+
+If an exclamation mark (`!`) is placed before a pattern's name, then characters
+are matched only when they _don't_ match the pattern. For example, `{!alpha}`
+will match all characters _except_ alphabetic ones.
+
+## Interpolating Text and Escaping
+
+To escape a character in a pattern (e.g. if you want to match the literal
+character `?`), you can use the syntax `{1 ?}`. This is almost never necessary
+unless you have text that looks like a Tomo text pattern and has something like
+`{` or `(?)` inside it.
+
+However, if you're trying to do an exact match of arbitrary text values, you'll
+want to have the text automatically escaped. Fortunately, Tomo's injection-safe
+DSL text interpolation supports automatic text escaping. This means that if you
+use text interpolation with the `$` sign to insert a text value, the value will
+be automatically escaped using the `{1 ?}` rule described above:
+
+```tomo
+# Risk of code injection (would cause an error because 'xxx' is not a valid
+# pattern name:
+>> user_input := get_user_input()
+= "{xxx}"
+
+# Interpolation automatically escapes:
+>> $/$user_input/
+= $/{1{}..xxx}/
+
+# This is: `{ 1{ }` (one open brace) followed by the literal text "..xxx}"
+
+# No error:
+>> some_text:find($/$user_input/)
+= 0
+```
+
+If you prefer, you can also use this to insert literal characters:
+
+```tomo
+>> $/literal $"{..}"/
+= $/literal {1{}..}/
+```
+
+## Repetitions
+
+By default, named patterns match 1 or more repetitions, but you can specify how
+many repetitions you want by putting a number or range of numbers first using
+`n` (exactly `n` repetitions), `n-m` (between `n` and `m` repetitions), or `n+`
+(`n` or more repetitions):
+
+```
+{4-5 alpha}
+0x{hex}
+{4 digit}-{2 digit}-{2 digit}
+{2+ space}
+{0-1 question mark}
+```
+