Enable optional experimental TUI via Trogon

[sullivancolin]

This PR adds the optional ability use a TUI via the Trogon project. It follows the same pattern as sqlite_utils by making it an optional dependency.

• trogon can be installed either via llm install trogon or during the installation of llm itself as in pipx install 'llm[tui]'.

• Using cog automatically adds the tui subcommand to the documentation. An additional section is added to usage.md mimicking the language used in sqlite_utils. There isn't currently any screen shot as part of the documentation like there is with sqlite_utils.

• The cog.yml and test.yml github action workflows now both also install trogon as well.

Sample Screenshot