Add autocompletions for `vf new -p` and `vf upgrade -p` with asdf or pyenv installs

[sbaack]

This commit adds autocompletions for vf new -p and vf upgrade -p to list version numbers from Python installations in asdf or pyenv. Those autocompletions are off by default and can be activated by setting a universal variable named VIRTUALFISH_PYVERSION_COMPLETION to either asdf or pyenv.

Note that this only seems to work with Fish's universal variable, I guess because virtualfish's autocompletions are loaded before config.fish is sourced. Also note that turning this on will deactivate autocompletions of files and directories for vf new -p and vf upgrade -p, but I think it's an acceptable trade-off.