Extract more rust-script logic into library convenient to use by rust-analyzer

[gwpl]

I made issue asking rust-analyzer to support rust-script , so I would benefit from better Vim support for rust-scripts in the end...

On issue concern, seemed like they would need to replicate rust-script logic: https://github.com/rust-analyzer/rust-analyzer/issues/9609#issuecomment-881305746

Maybe there is option for two project to collaborate and find way how to make it work? E.g. rust-script exporting some logic into libraries convenient to use by rust-analyzer?

[fornwall]

In general, I think it makes sense to extract functionality into library code here.