SE-0351 was accepted with `mapOutput`, but it's not implemented

[tevelee]

The proposal for SE-0351 defines a way to map the output of a regex component to reorder captures, flatten nested optionals, or create instances of a custom type. See https://github.com/apple/swift-evolution/blob/main/proposals/0351-regex-builder.md#mapping-output
The proposal was accepted and its status is declared as Implemented (Swift 5.7), but this feature was left out from the compiler.

This is a crucial feature, and I miss it a lot whenever I deal with regexes in Swift. It's not possible to implement it as an extension by external users of the API, because it should have been a builtin component in the DSL tree.

I only found traces of it in the codebase, the PR #455 that implemented it was closed and the follow-up #466 is still in draft.

[tevelee]

cc @milseman

[sena-mike]

This pretty much flattens out all regexes to a single level and limits how much composition can be done into custom types. Hope this gets some attention for the next release of Swift

[haikusw]

Want this also. Nested tuples to support nested Captures would be better than nothing but a way to capture and return custom types to group multiple captures into structs would be better.