Add interpreter command

[Gitznik]

• I have added a news fragment under changelog.d/ (if the patch affects the end users)

Summary of changes

Add a interpreter command with list and prune subcommand. They can be used to list the installed standalone interpreters and prune unused standalone interpreters.
Resolves #1248

Test plan

Tested by running

pipx install --fetch-missing-python --python 3.7 pycowsay
pipx interpreter list
pipx uninstall pycowsay
pipx interpreter prune

[dukecat0]

Besides, running pipx interpreter without any arguments will result in an error:

Unknown interpreter command None

[Gitznik]

Besides, running pipx interpreter without any arguments will result in an error:

Unknown interpreter command None

I've now adapted the logic from running pipx without a subcommand. Unfortunately that means passing around the subparser, to be able to call print_help on it. There is no public API to retrieve the subparsers from the main parser unfortunately.

[chrysle]

Thank you for this, good work! Sorry for the lack of review here.