Allow to target a setting in a workspace file to one of the roots

[gjsjohnmurray]

I had discussions about this with others from the team in the past, even in the context of #45177 and I think what is missing is some variable you can use as part of the glob pattern to point to one of the entries in the workspace file. If someone has good ideas how to do that, we can have a discussion in a separate issue.

Originally posted by @bpasero in #4873 (comment)

A previous suggestion from @roblourens is at #70438 (comment)

This aligns with the existing feature for scoping variables per workspace folder:

https://code.visualstudio.com/docs/editor/variables-reference#_variables-scoped-per-workspace-folder

[bpasero]

In my own words and from my ideas in #45177 (comment): Today you cannot easily reference one of the roots of a .code-workspace file from within that file. The use case is that you might want to craft a glob pattern in a setting such as files.readonlyInclude to target a specific root of the workspace without having to use an absolute path.

//cc @sandy081

[bpasero]

It seems like the C++ extension supports the following syntax from a .code-workspace file:

"C_Cpp.default.includePath": [
  "${workspaceFolder:cna-mme_s1}/**",
  "${workspaceFolder:cna-mme_s1}/working/s1/config/",
]

I think we can think about expanding this syntax to more settings that are about paths.

@sandy081 do you know if this is something we support built-in to core or is this something the C++ extension implemented only for their use case?

[sandy081]

We do not support variables in settings. At present the owner of the setting should resolve itself - #2809