Add mackup command

[twpayne]

Refs #885 and lra/mackup#1733.

First steps. Not ready for merge yet.

[twpayne]

@MurzNN this PR adds

$ chezmoi mackup add $APPLICATION

where $APPLICATION is the basename of the file that you'll find in /opt/homebrew/Cellar/mackup/0.8.32/libexec/lib/python3.9/site-packages/mackup/applications, for example curl or vscode. This assumes that you have installed mackup on your machine with brew and that your Python version is 3.9.something.

Right now this is now an undocumented command in chezmoi. It should work, but what's needed is:

1. Agreement from the mackup developers that it's OK to use their config files in this way.
2. Fewer assumptions about where mackup stores its config flies. Right now, this PR assumes the use of Python 3.9, but it's not a robust assumption. Ideally, the mackup developers would add a command that prints the directory name of where it stores its config files.
3. (lower priority) Deeper integration with mackup, e.g. list and unmanage commands.

[twpayne]

@ecogit @drod3763 @realdimas you all 👍 'd the original issue (#885). Do you have any feedback on this implementation?

[drod3763]

I don't use mackup, I just thought the templates were a good idea.