Stealth: CSS Parser Integration

[cookiengineer]

The CSS parser is now almost ready to be used in practice. In order to integrate it with the Browser UI, it needs the following features (that are still missing):

• [ ] Implement attr() support in parse_value()

• [ ] Implement calc() support in parse_value()

• [ ] Implement hsl/a color support in parse_value() and convert colors to rgba format

• [ ] Implement CSS.isCSS() method that verifies CSS tree structure, similar to parser/IP and parser/URL's behaviours.

Afterwards, the CSS parser requires render() integration, so that the CSS Optimizer can parse and render the filtered CSS files correctly:

• [ ] Migrate NORMAL.mjs to parse/render signature for each property.
• [ ] Migrate SHORTHAND.mjs to parse/render signature for each property.