When entering -webkit-text-stroke in the advanced style section, it was
incorrectly converted to --text-stroke (CSS variable). This happened because
normalizeProperty() strips vendor prefixes from properties not in the
prefixedProperties allowlist, and then parse-style-input.ts sees the unknown
property 'text-stroke' and converts it to a CSS variable.

Changes:
- Add -webkit-text-stroke, -webkit-text-stroke-color, and
  -webkit-text-stroke-width to prefixedProperties list
- Add test for -webkit-text-stroke preservation