Because the `token_chunks` need to pre-visit tokens and collect, it
could be done at add tokens phase. So here export it let rolldown could
be improve `renderChunks` sourcemap encode.
The sourcemap implement port from
[rust-sourcemap](https://github.com/getsentry/rust-sourcemap), but has
some different with it.
- Encode sourcemap at parallel, including quote `sourceContent` and
encode token to `vlq` mappings.
- Avoid `Sourcemap` some methods overhead, like `SourceMap::tokens()`
caused extra overhead at common cases. Here using `SourceViewToken` to
instead of it.
