c_cpp_properties.json: Support ${workspaceFolder} substitution

[seanmlyons22]

c_cpp_properties.json supports vscode variables such as ${workspaceFolder}. However, this must be translated to something that LLVM can understand.
I propose a small enhancement to the CCppProperties class where ECC will attempt to replace ${workspaceFolder} with . in the includePath list. According to this issue it seems that assuming that .vscode/c_cpp_properties.json is located in the root folder is a safe assumption.

I don't believe this would impact any users who do not have ${workspaceFolder} in their c_cpp_properties file.

Create minimal example if you can

An example of a json that I would like to add support for:

{
  "version": 4,
  "configurations": [
    {
      "name": "Win32",
      "includePath": [
        "${workspaceFolder}/source",
      ],
      "defines": [
      ],
    },
  ]
}

I would be willing to open a PR for this if acceptable. I tested the following change and it worked for my use case.
If you're interested, I can make some tests and formalize a PR:

        def parse_includes_from_json(content):
            try:
                include_paths = content["configurations"][0]["includePath"]
            except Exception:
                include_paths = []

            # VSCode variable for current working directory
            vsc_ws = '${workspaceFolder}'
            # Attempt to replace VSCode variables if any
            includes = [i.replace(vsc_ws, '.') for i in include_paths]
            includes = [path.expandvars(i) for i in include_paths]
            includes = ["-I{}".format(include) for include in includes]
            return includes

[niosus]

I was not the one who implemented the support for c_cpp_properties.json, so if you think it can work better, I am happy to have a look at a PR and merge it eventually, but I will not be able to provide much guidance apart from checking how it plays together with the other parts of the code.

[mclayton7]

@niosus @seanmlyons22 I think this sounds reasonable. There should be some tests that'll help verify the implementation, but if you have any questions about the existing implementation, don't hesitate to @ me!

[niosus]

Thanks for chiming in @mclayton7, really appreciate this! @seanmlyons22 feel free to implement what you consider makes sense, open a PR and ping both me and @mclayton7 on it. This way we can go forward with this.

[stale]

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