Support both `r`, `g`, `b` and `R`, `G`, `B` for displaying images

[LucaMarconato]

IMHO, since intuitively acronyms are associated with upper-case initials, I would make this case-insensitive.

Originally posted by @gboscagli in #371

In napari-spatialdata we differentiate between multichannel images and RGB images via the name of the channels. When channels are r, g, b they are interpreted as RGB images. @gboscagli suggests to make this check case-insensitive.

Tagging devs working on other viewers. WTYT? @timtreis @Sonja-Stockhaus @keller-mark

I'm fine making it case-insensitive.