[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`.

Author: rxwei