Support for Nested Selectors, Nested Declarations and At-Rule Validations

[acemir]

This PR enhance the parser's ability to correctly process complex CSS structures, including nested selectors, nested declarations and at-rules with conditions.

Key Changes

Introducing CSS Nesting Module support

• Added support for CSSNestedDeclarations
• CSSStyleRule now inherits properties from CSSGroupingRule

Improved At-Rule Validation

• Refactored at-rule validation logic ensuring that invalid or malformed at-rules are ignored more reliably.
• Improved handling of edge cases, such as unbalanced braces and complex at-rule conditions.
• Enhanced handling of at-rules that cannot be nested.

Regex and Comment Clarifications

• Updated and clarified several regular expressions and their associated comments for better maintainability and understanding.
• Improved documentation for regexes used in whitespace and at-rule validation.

Test Improvements

• Added tests for CSS Nesting and At-Rule Validations
• Added failing tests for currently unsupported rules like CSSLayerStatementRule and imports with layer and supports

Motivation

These changes address issues with the parser's handling of nested CSS blocks and at-rules, making it more reliable with the current state of the browser's CSS implementations.

Impact

• More accurate parsing of complex and nested CSS.
• Better handling of invalid rules.
• Improved code readability and maintainability.

[changeset-bot]

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Latest commit: 9cae1fc

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[acemir]

Please, review @YunFeng0817