Add support for specifying separate C and C++ compiler paths and args

Feature Request

Currently, we have compilerPath and compilerArgs fields in c_cpp_properties.json. Arguments (i.e. -std=c++20 vs -std=c17), and perhaps even the compiler itself (i.e. XC16), can be C or C++ specific. These field may be insufficient for users who want to mix C and C++ sources. The same issue exists for fields in custom configuration provider interfaces.

This feature request tracks separating compilerPath and compilerArgs into C and C++, so separate values for each can be provided. One backwards compatible approach would be to allow the existing fields to be overloaded with an object containing a separate value per language. For example:

"compilerPath": {
    "c": "/usr/bin/gcc",
    "cpp": "/usr/bin/g++"
},
"compilerArgs": {
    "c": [ "-std=c17" ],
    "cpp": [ "-std=c++20" ]
}

Related: https://github.com/microsoft/vscode-cpptools/issues/7534

Start with the c_cpp_properties.json fields and the custom configuration provider interfaces described in the issue. Determine how separate C and C++ compiler paths and arguments can be represented while preserving existing fields; done means C and C++ sources can receive distinct values with backward compatibility.