Allow pointing `SourceFileConfigurationItem` to a compile_commands.json file.

Feature Request

When implementing a CustomConfigurationProvider for intellisense, it would be good to be able to construct a SourceFileConfigurationItem from an existing compile_commands.json.

At Arm we're developing an extension where different files in a project could belong to different build "contexts" e.g. for a different target or compiler toolchain, so we need to be able to set different intellisense configurations depending on which file is being viewed.

Currently I am trying to parse out the command portion of the compile_commands.json to get the compiler args/fragments, and building up the compilerFragments list, but it would be so much better to point at compile_commands.json directly, similar to how it is being done in the c_cpp_properties.json config.

I hope this makes sense. If you have any alternative ideas on how to achieve this, it would be great to hear about it.

Thanks!

SourceFileConfigurationItem API と CustomConfigurationProvider API から始め、issue で言及されている c_cpp_properties.json の処理と現在の動作を比較します。compile_commands.json ファイルがファイルごとの IntelliSense 構成をどのように提供すべきかを定義し、期待される API とテストをメンテナーと確認します。呼び出し元がコンパイラーコマンドを手動で解析せずにファイルを指定できれば完了です。