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!

从 SourceFileConfigurationItem 和 CustomConfigurationProvider API 入手，然后将它们当前的行为与 issue 中提到的 c_cpp_properties.json 处理方式进行比较。确定 compile_commands.json 文件应如何提供每个文件的 IntelliSense 配置，并与维护者确认预期的 API 和测试；当调用方无需手动解析编译器命令即可指向该文件时，即表示完成。