Provide a public API to set the environment to a given path

[brettcannon]

I have an extension that currently changes the python executable being used by setting it in the user or workspace settings.json (depending how the user chooses to do it).

In the description of https://github.com/microsoft/vscode-python/wiki/AB-Experiments#deprecatepythonpath, I can't really see any way for an extension to update the python used by the vscode-python extension after the move to stop using the settings.json.

So, this is a request to provide an API/command to provide parity with what was already available prior to this change (or if there is already a way to do it, document it in https://github.com/microsoft/vscode-python/wiki/AB-Experiments#deprecatepythonpath).

Originally posted by @fabioz in a discussion