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!

Start with the SourceFileConfigurationItem and CustomConfigurationProvider APIs, then compare their current behavior with the c_cpp_properties.json handling mentioned in the issue. Define how a compile_commands.json file should provide per-file IntelliSense configurations and confirm the expected API and tests with maintainers; done means callers can point to the file without manually parsing compiler commands.