Improve checks for invalid CSS function names

Currently, it's possible for users to define functions whose names differ from reserved names only by case, such as EXPRESSION() or URL(). These functions can't be invoked normally, because the logic for function calls is case-insensitive, but unlike their lowercase equivalents they're not forbidden at definition-time. We should make the invalid function name check case-insensitive to deprecate and eventually forbid these names.

Edit: I've updated the title to broaden the scope here a bit. There are a number of improvements to be made to this check, mostly in the form of loosening it for cases that aren't (or are no longer) of real concern.

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• Proposal (https://github.com/sass/sass/pull/4053)

Deprecation

• Tests (https://github.com/sass/sass-spec/pull/2109)
• Implementation (https://github.com/sass/dart-sass/pull/2731)
• Documentation (https://github.com/sass/sass-site/pull/1527)
• Update canonical spec (https://github.com/sass/sass/pull/4186)

Final Changes

• Tests
• Implementation
• Documentation