Update SCSS and CSS syntax highlight

Changes:
- Use less and more simple context rules.
- Support complex SCSS structures, e.g. variables in @media, nesting, variable define.
- Support SCSS functions.
- Support CSS @keyframes structure and @media conditions.
- Different highlight colors for id, class, attribute and pseudo selectors.
- Reduce highlight errors.

FIXED-IN: KDE Frameworks 5.32
REVIEW: 128925
BUG: 368730
BUG: 376005