[Feat Request]Make `SourceMap` struct is `Sync`.

[underfin]

The sourcemap crate is amazing, it maybe used extensive at rust js tools, but here has a problem. The rust js tools always run at parallel(oxc and swc codegen), but the SourceMap struct is not Sync because SourceView using Refcell. Here has a workaround encode it to String map, but after it need to decode to SourceMap, it has an unnecessary overhead. Hope get your advice, thank you.