fix: mixin with native CSS nesting

[idoros]

This PR fixes some cases of mixins with native CSS nesting:

• mixin with nesting rules
• mix into a nested rule
• preserve nesting selector inside mixed fragment
• error diagnostic in case of a mixin class that is found inside a nested fragment (temporary until we figure out how to handle this case better)
• mix nested at-rule with content
• preserve nesting rules and declaration validity when mixed in an invalid order (nest declarations following nested rules with & selector)