Support solution filters (*.slnf)

[JoeRobich]

based on #4076

[codecov]

Codecov Report

Merging #4481 (5e8f728) into master (b6d16ba) will increase coverage by 0.02%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #4481      +/-   ##
==========================================
+ Coverage   68.96%   68.98%   +0.02%     
==========================================
  Files         109      109              
  Lines        4282     4282              
  Branches      741      741              
==========================================
+ Hits         2953     2954       +1     
+ Misses       1124     1123       -1     
  Partials      205      205              

Flag	Coverage Δ
integration	52.59% <50.00%> (+0.02%)	⬆️
unit	85.81% <ø> (ø)

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Impacted Files	Coverage Δ
src/configurationProvider.ts	22.64% <ø> (ø)
src/omnisharp/server.ts	66.50% <0.00%> (ø)
src/omnisharp/launcher.ts	55.69% <100.00%> (ø)
src/features/codeLensProvider.ts	54.54% <0.00%> (+1.29%)	⬆️

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