styles: case-insensitive style registration and lookup

[jmooring]

Normalize style names to lowercase during registration and lookup to provide case-insensitive access across the library, CLI, and web interface.

Changes:

• Normalize keys to lowercase in styles.Register().
• Convert input names to lowercase in styles.Get().
• Update cmd/chroma and cmd/chromad to use these normalized keys.
• Add regression tests for library and CLI case-insensitivity.
• Update CI workflow to include tests for the cmd/chroma module.
• Update README.md to document case-insensitive style names.

Closes #1222

[jmooring]

I recognize that this may appear to be a broad approach to resolving #1222, but there are a few key reasons for the design:

• It provides backwards compatibility, ensuring that all existing scripts and integrations continue to work regardless of how they reference style names.
• With the exception of the recent "RPGLE" addition, Chroma has already standardized on lowercase style names. This PR simply implements that convention formally in the code to prevent future inconsistencies.
• I've also added the first set of regression tests for the CLI and updated the CI workflow to ensure the cmd/chroma module is actually being tested moving forward.

This ensures that the library, CLI, and web interface all behave consistently.

[alecthomas]

Love it!