[Proof of concept] Support shell autocomplete for templates

[pneff]

Together with click's Shell Completion support, this allows to auto-complete llm -t <tab> invocations.

This is an initial proof of concept. If you think this is the right thing to do, I can invest a little time to polish it. A few tasks I see as open:

• Check which other arguments would benefit from such treatment.
• Extend the documentation to include the autocomplete setup in the user guide.
• Fix auto-completion on the initial command. For example llm <tab> currently doesn't work.
• Performance is an issue, it's rather slow on my end.

For information how to set up the autocomplete here, see the official Click Shell Completion documentation.

[pneff]

I have extended this slightly to auto-complete models as well. Template auto-complete also is fixed now to actually complete based on partial template names.

For command completion I sources the root cause, see click-contrib/click-default-group#22.

@simonw let me know if you are happy for me to continue with this PR. I would then add the documentation and some test coverage here.