feat(extra): add Gemini CLI theme

[danrneal]

Description

This PR adds support for generating Gemini CLI (https://github.com/google-gemini/gemini-cli) themes. It provides an extra template that maps the Tokyo Night palette to the custom theme format used by the CLI, supporting all four styles (Storm, Moon, Night, and Day).

The implementation includes:

• Comprehensive Mapping: Maps Tokyo Night colors to Gemini CLI's syntax highlighting and modern semantic UI tokens.
• Diff Support: Utilizes the pre-blended colors.diff background shades for improved readability in terminal diff views.
• Documentation: Includes a README in extras/gemini_cli/ with instructions for sideloading the generated JSON files.

Screenshots

[danrneal]

Hi @folke , just a polite bump on this.

This adds a gemini-cli extra using the standard template system. I've verified the generated themes against the CLI to ensure the syntax highlighting and diff colors render correctly for all four styles.

Happy to make any adjustments if needed!