Drop support for misplaced rest arguments

Currently, Dart Sass allows rest arguments to appear anywhere in an argument invocation, even though they're only ever evaluated last and placed after all positional arguments. While this is contrary to the specified behavior, it's been supported back through Ruby Sass, and so we should go through a proper deprecation process before removing it entirely.

Deprecation

• Tests (https://github.com/sass/sass-spec/pull/2072)
• Implementation (https://github.com/sass/dart-sass/pull/2623)
• Documentation (https://github.com/sass/sass-site/pull/1447)
• Add deprecation metadata (https://github.com/sass/sass/pull/4121)

Removal

• Language
• Tests
• Implementation
• Documentation