Forbid multiple !global and/or !default flags

• Deprecation
  • Tests (https://github.com/sass/sass-spec/pull/1897)
  • Dart Sass (https://github.com/sass/dart-sass/pull/1931)
  • Sass Site (https://github.com/sass/sass-site/pull/716)
• Removal
  • Deprecation metadata (https://github.com/sass/sass/pull/4129)
  • Tests (https://github.com/sass/sass-spec/pull/2078)
  • Dart Sass (https://github.com/sass/dart-sass/pull/2636)
  • Website

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Currently, variable assignments can have any number of !default or !global flags. Multiple instances of the same flag don't do anything, though. They should be rejected by implementations.

Since this is technically a breaking change, it will need a deprecation period. The spec already requires at most one of each flag, so it won't need to be updated.