Expose scope and source_map for use as library in GUI editor

[rummatee]

Hi @oxalica

I am building a graphical editor for Nix and am using the amazing syntax and ide crates from nil to parse and analyze the nix expressions.
I want to let users choose from the identifiers in scope, so I need to be able to list them.
Your Analysis struct already uses this information, but doesn't expose it publicly, so I would like to add some small scopes and source_map functions to retrieve them.
Would you be willing to support this use case and consider a PR? I'm happy to adjust it to any requirement you might have.

Thanks in advance for considering my request!