interface for the colr font feature?

[u-fischer]

Description

As shown here at least lualatex can make use of colored fonts:

https://tex.stackexchange.com/questions/520182/is-there-any-tex-based-engine-capable-of-rendering-the-colored-font/520183#520183

https://tex.stackexchange.com/a/387069/2388

https://tex.stackexchange.com/questions/497403/how-to-use-noto-color-emoji-with-lualatex

It would be nice to have a fontspec interface.

(Currently luaotfload detects colr but activates it only by default in harf mode. This is a bit inconsequent, we probably will change this in the one or the other way).