Removed Porposal Attributes from everywhere

[david-driscoll]

Spec 3.16 has been released!

Namespaces have changed and Proposals has been removed.

Introduced a new Proposals assembly for both DAP and LSP, these assemblies will house any future proposal classes, this should help avoid making breaking changes to the current protocol versions.

[codecov]

Codecov Report

Merging #480 (4aebc8e) into master (2055eaa) will increase coverage by 0.34%.
The diff coverage is 95.91%.

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
+ Coverage   72.73%   73.07%   +0.34%     
==========================================
  Files         254      256       +2     
  Lines       12278    12246      -32     
  Branches      840      838       -2     
==========================================
+ Hits         8930     8949      +19     
+ Misses       3348     3297      -51     

Impacted Files	Coverage Δ
src/Client/LanguageClientOptionsExtensions.cs	42.70% <ø> (ø)
src/Server/LanguageServerOptionsExtensions.cs	41.66% <ø> (ø)
...rc/Server/Pipelines/SemanticTokensDeltaPipeline.cs	100.00% <ø> (ø)
...als/Serialization/ProposedCapabilitiesConverter.cs	83.33% <83.33%> (ø)
...l.Proposals/Serialization/ProposedLspSerializer.cs	93.33% <93.33%> (ø)
src/Client/LanguageClient.cs	97.24% <100.00%> (+0.01%)	⬆️
...lient/LanguageClientServiceCollectionExtensions.cs	97.16% <100.00%> (+0.02%)	⬆️
src/JsonRpc/JsonRpcServerOptionsBase.cs	78.48% <100.00%> (ø)
src/JsonRpc/Serialization/SerializerBase.cs	96.77% <100.00%> (+0.77%)	⬆️
src/Protocol.Proposals/ProposedExtensions.cs	100.00% <100.00%> (ø)
... and 9 more

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