[SE-0351] Revise regex builder proposal (#1634)

* [SE-0351] Revise regex builder proposal

Revision:
- Capture takes throwing closures. (swiftlang/swift-experimental-string-processing#261)
- Rename `Output` associated type to `RegexOutput`. (swiftlang/swift-experimental-string-processing#281)
- Define primary associated type for `RegexComponent`.

Clarification:
- Clarify that `RegexComponent` and `Regex: RegexComponent` will be in the stdlib, not in `RegexBuilder`.
- Make detailed design driven by example and move API definition to a collapsible. This is so that the complex result builder machinery won't obscure the API design.
- Add alternative considered section about unifying `Capture` and `TryCapture`.

* Add section about anchors and lookaheads

* Add section re `mapOutput`

* Move mapOutput to Regex instead of RegexComponent

* Address comments.

* Minor fix in RegexComponent declaration

* Capture transform shouldn't be restricted to taking a `Substring`

* Fix syntax highlighting

* Update 0351-regex-builder.md

* Revise `RegexComponentBuilder.buildPartialBlock(first:)`.

Also fix broken links.

* Update 0351-regex-builder.md

Co-authored-by: Nate Cook <[email protected]>
Co-authored-by: Stephen Canon <[email protected]>
Co-authored-by: Ben Cohen <[email protected]>

Author: 4 people