Use .python-version to select python interpreter

[tekumara]

pyenv uses a .python-version file when selecting the python version. It would be super nice if the workspace Python Interpreter selection would use .python-version to select the appropriate pyenv version on a system with pyenv.

Current behaviour

1. If python.defaultInterpreterPath is set, this will be the workspace's python interpreter when opening a workspace containing a .python-version file, rather than the version specified in .python-version which might differ.
2. When opening a terminal, if the selected python interpreter is a pyenv intrepreter, the pyenvActivationProvider will run pyenv shell to set the terminal to the workspace's python interpreter. This may differ from the version in .python-version. This conflicts with the behaviour of pyenv in a non-vscode terminal.

[karthiknadig]

Duplicate #1167

This is being worked on in the current iteration. #22669