Deprecate and remove meta.feature-exists()

[nex3]

From sass/sass-spec#1939 (comment):

In practice, essentially all the new features we add fall into one of three categories:

1. New built-in functions or mixins, which are easy to detect using other sass:meta functions.
2. Language-level features which are relatively easy to detect on their own (for example, first-class calc can be detected with calc(1 + 1) == 2).
3. New syntax which can't even be parsed in implementations that don't support it, for which feature detection isn't particularly useful anyway.

This, combined with the fact that we haven't added a new feature to meta.feature-exists() in years, suggests that it's not useful anymore and should be removed.

[nex3]

#3749 has landed and the proposal is out for review. Since it's a breaking change, I'll give it a month for public comment before we can mark it as accepted and implement.

[nex3]

@stof We just released 1.77.0, so these will need to be 1.78.0.

[stof]

@nex3 I updated all PRs to mention 1.78.0, and I also opened the missing PR for the website to document the deprecation.