Add shell completion for models, templates and option names

[yorickvP]

This adds support for autocompleting the -m, -t and -o arguments. I've found it very useful so far to look up gemini models and remember what the options were called.

click shell_completion docs: https://click.palletsprojects.com/en/stable/shell-completion/

Quick setup/testing:

• bash: eval $(_LLM_COMPLETE=bash_source llm)
• zsh: eval "$(_FOO_BAR_COMPLETE=zsh_source foo-bar)"
• fish: _LLM_COMPLETE=fish_source llm | source