feature(lsp): adds document symbol provider

[baszalmstra]

Adds an LSP document symbol provider. The result is based on the AST of a SourceFile. The implementation is adapted from the rust-analyzer code.

This is a take 2 it depends on #295 . Its a much cleaner approach using the previous PRs. It supersedes #293 .

[codecov]

Codecov Report

Merging #297 (0bbdbd4) into master (6081592) will increase coverage by 0.64%.
The diff coverage is 97.66%.

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
+ Coverage   80.77%   81.42%   +0.64%     
==========================================
  Files         237      239       +2     
  Lines       13864    14026     +162     
==========================================
+ Hits        11199    11420     +221     
+ Misses       2665     2606      -59     

Impacted Files	Coverage Δ
crates/mun_language_server/src/capabilities.rs	0.00% <0.00%> (ø)
crates/mun_language_server/src/lib.rs	11.11% <ø> (+4.44%)	⬆️
crates/mun_syntax/src/lib.rs	63.04% <ø> (ø)
...n_language_server/src/state/protocol/dispatcher.rs	48.52% <87.50%> (+13.98%)	⬆️
crates/mun_language_server/src/state.rs	88.41% <92.30%> (+20.20%)	⬆️
crates/mun_language_server/src/analysis.rs	100.00% <100.00%> (ø)
crates/mun_language_server/src/conversion.rs	50.00% <100.00%> (+27.77%)	⬆️
crates/mun_language_server/src/file_structure.rs	100.00% <100.00%> (ø)
crates/mun_language_server/src/handlers.rs	100.00% <100.00%> (ø)
crates/mun_language_server/src/state/protocol.rs	39.43% <100.00%> (+0.86%)	⬆️
... and 13 more

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[Wodann]

Up to you to decide whether you think it's necessary to rename on to on_request