feat(lsp): support for completions

[baszalmstra]

Adds initial support for completions for IDEs. This is far from finished, there is a lot more to add but I didn't want to make this too big. Currently supports two types of completion:

• Completion of names in scope
• Completion of struct fields

I used the same design as Rust Analyzer.

This PR depends on #298

[codecov]

Codecov Report

Merging #306 (b1f50b2) into master (09ded01) will decrease coverage by 0.06%.
The diff coverage is 73.85%.

❗ Current head b1f50b2 differs from pull request most recent head 012fa9e. Consider uploading reports for the commit 012fa9e to get more accurate results

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
- Coverage   80.86%   80.80%   -0.07%     
==========================================
  Files         245      258      +13     
  Lines       14937    15480     +543     
==========================================
+ Hits        12079    12508     +429     
- Misses       2858     2972     +114     

Impacted Files	Coverage Δ
crates/mun_hir/src/code_model.rs	11.11% <0.00%> (ø)
crates/mun_hir/src/code_model/src.rs	43.33% <ø> (ø)
crates/mun_hir/src/item_scope.rs	49.36% <ø> (ø)
crates/mun_language_server/src/analysis.rs	82.60% <0.00%> (-17.40%)	⬇️
crates/mun_language_server/src/capabilities.rs	0.00% <0.00%> (ø)
crates/mun_language_server/src/from_lsp.rs	76.19% <0.00%> (-23.81%)	⬇️
crates/mun_language_server/src/handlers.rs	75.60% <0.00%> (-24.40%)	⬇️
crates/mun_language_server/src/lib.rs	11.11% <ø> (ø)
crates/mun_language_server/src/to_lsp.rs	37.50% <0.00%> (-26.14%)	⬇️
crates/mun_language_server/src/completion/item.rs	67.74% <67.74%> (ø)
... and 47 more

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[Wodann]

Generally looks good. It's a lot of complex code though, so I didn't comprehend it all. I noticed that quite a lot of code was untested. As this is (seemingly) complex code, it'd be good to add a broader suite of tests (as we keep expanding on our LSP.

[Wodann]

Mun Rulezz!

[baszalmstra]

"intellijRulezz" is a standard string used for this purpose