Removed `is_rgb` in `fmt 11.2.0`

[Hind-M]

Hello,

This PR optimizes the text_style and removes is_rgb (starting from 11.2.0 release).
How should we replace is_rgb in our codebase, should it be !is_terminal_color() as replaced in here?
Thanks!

[vitaut]

!is_terminal_color() is indeed the replacement for is_rgb but this API is mostly for implementing formatters and I wouldn't recommend using it in your code.