Support system variable substitution in VSCode extension config

It'd be nice to support system variable substitution (like `${workspaceFolder}`) in VSCode extension config, especially when using custom `rootDirectory` and `pathToRelay`.
While VSCode doesn't provide any API for this, [`vscode-python` does support this feature by reimplementing the substitution system by hand](https://github.com/microsoft/vscode-python/blob/b68facc42f684f26c5f83f3cfe895e7aa4bb2bee/src/client/common/variables/systemVariables.ts).
If it's okay to bring that piece of code directly into our extension, I'm open to contributing that one.