Implement de/serialization for Sources

[dylanowen]

Maybe there is a better way to do this but I'd like to serialize/deserialize the Sources (either with serde or musli). What I have setup is Unit compilation and serialization to disk but I'm running into the issue that my errors would be better with the original Sources.

I would just recreate the sources as needed but I believe the SourceIds are generated based on how the SourceLoader is called which means I'd basically need to rerun compilation on error. If I could de/serialize the sources then I could just read the sources that were originally used for compilation.