Deprecate and remove meta.feature-exists()

• Proposal (https://github.com/sass/sass/pull/3749)
• Deprecation
  • Update official spec (https://github.com/sass/sass/pull/3860)
  • Dart Sass (https://github.com/sass/dart-sass/pull/2235/)
  • Tests (https://github.com/sass/sass-spec/pull/1986)
  • Website (https://github.com/sass/sass-site/pull/1148)
• Removal
  • Update official spec (https://github.com/sass/sass/pull/4230)
  • Dart Sass (https://github.com/sass/dart-sass/pull/2781)
  • Tests (https://github.com/sass/sass-spec/pull/2141)
  • Website

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From https://github.com/sass/sass-spec/issues/1939#issuecomment-1738040431:

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.