Add multiroot-workspace variable scoping for `python.defaultInterpreterPath`

It’s currently not possible to use multiroot-workspace variable scoping to configure python.defaultInterpreterPath, as @kimadeline mentioned in https://github.com/microsoft/vscode-python/issues/18207#issuecomment-1055840598.

My use case is the following. I have multiple python projects in my workspace but I want them all to use the same virtualenv, existing under one of the workspace folders. The tree structure looks like this:

/
├── python-a/
│  ├── .venv/
│  └── foo.py
├── python-b/
│  └── bar.py

To make it work with python.defaultInterpreterPath I need to configure it like this in the workspace file:

"python.defaultInterpreterPath": "${workspaceFolder}/../python-a/.venv/bin/python"

This is fragile, as it will break if python-b/ workspace folder changes its absolute path. With multiroot-workspace variable scoping it would be possible to express this dependency more robustly like this:

"python.defaultInterpreterPath": "${workspaceFolder:python-a}/.venv/bin/python"

[karrtikr]

Hi all, feature should be out in the pre-release version of the extension, use the following to try it out:

[brettcannon]

Thank you to everyone who upvoted this issue! Since the community showed interest in this feature request we will leave this issue open as something to consider implementing at some point in the future.

We do encourage people to continue 👍 the first/opening comment as it helps us prioritize our work based on what the community seems to want the most.