Support emitting source map for `Joiner`.

[hyf0]

Won't be hard and just port https://github.com/Rich-Harris/magic-string/blob/6f6cd52270fdc8b62b1b94c73a5d19ba37b3d4dd/src/Bundle.js#L85-L167 would be enough.

See https://github.com/rolldown-rs/string_wizard/blob/00ff3ef9bfd9f63dd0d932bafa69a411717c84fa/src/magic_string/source_map.rs#L13 for references.

This will also meet the #4 problem, but we could ignore it so far. I will try solve #4 in one go.

[hyf0]

@underfin Do you have time to help solving this issue? Or I will solve it this weekend.

[underfin]

Please hold it, it is unnecessary at now.

[hyf0]

Ok. Get it.