[Feature Request] Add lowercase/case-insensitive highlighting as an option

[omeyenburg]

It would be practical to have an option to enable case-insensitive (or lowercase) highlighting for color names. This would allow lowercase (e.g., "blue") or uppercase (e.g., "BLUE") color names to be highlighted.

A similar feature was released for norcalli/nvim-colorizer.lua but not added to its config.