Fix CSS statement at-rules consuming following rules

- stop statement at-rules such as @charset at their semicolon
  - preserve the following selector and declaration block
  - support feature-level not() expressions in media queries
  - add regression coverage for @charset and negated media features

Fixes Lobsters theme variables being discarded, which caused mixed light and dark colors when loading its stylesheets.

include/eepp/ui/css/mediaquery.hpp

@@ -119,6 +119,7 @@ struct EE_API MediaQueryExpression {
 	float fval{ 0 };
 	float fval2{ 0 };
 	bool checkAsBool{ false };
+	bool negated{ false };
 	std::string valStr;
 
 	MediaQueryExpression();