Add named capturing groups to gleam/regex.Match

As it is now, you have the ability to write named capturing groups, but those results don't appear in the list of matches returned from `regex.scan`.

```gleam
import gleam/io
import gleam/regex

pub fn main() {
let assert Ok(regex) = regex.from_string("(?,)")
let content = "Hello, world!"

regex.scan(regex, content)
|> io.debug
}
```

```
[Match(content: ",", submatches: [Some(",")])]
```

I propose `Match` have the following signature:

```gleam
pub type Match {
Match(
/// The full string of the match.
content: String,
/// A `Regex` can have subpatterns, sup-parts that are in parentheses.
submatches: List(#(String, Option(String))),
)
}
```

So now running something like this

```gleam
import gleam/io
import gleam/regex

pub fn main() {
let assert Ok(regex) = regex.from_string("(?new|old)\\s+(\\w+)")
let content = "new match_type"

regex.scan(regex, content)
|> io.debug
}
```

would give you this

```
[Match(content: "new match_type", submatches: [#("type_of", Some("new")), #("2", Some("match_type"))])]
```

If the concern is this may break existing used regex, possibly a new added option (which will still break existing compiled regex) or a new `groups` function can be added that returns just a list of tuples of group name and submatches.