Add support for replacement of vscode variables

[seanmlyons22]

Convert Variables such as ${workspaceFolder} so they can be understood by LLVM

Close #741

[niosus]

@seanmlyons22 thanks for the contribution. I might be missing a point a bit though. This is already done by using other variables. For example ${project_base_path} will expand to a full path to the project root. How would this variable be different?

[seanmlyons22]

@niosus, sorry this got lost in my inbox. This would be exactly the same as the other variables. I will update this to be replaced in the same manner.

Edit: Looked at project_base_path, the difference between that variable and ${workspaceFolder} is that the latter is a VSCode variable, so it should only be replaced for c_cpp_properties.json, not for the others.

[niosus]

Got it. Makes sense to me!