Fix: deduplicate unparsed -webkit-mask declarations

[franciscolourenco]

-webkit-mask is currently duplicated when a rule contains an authored prefix pair with a value it can't fully parse (e.g. containing var()):

.foo {
  -webkit-mask: url(mask.svg) var(--offset);
  mask: url(mask.svg) var(--offset);
}

Resulting in:

.foo {
  -webkit-mask: url(mask.svg) var(--offset);
  -webkit-mask: url(mask.svg) var(--offset);
  mask: url(mask.svg) var(--offset);
}

With targets that require prefixing mask, -webkit-mask gets duplicated when the author already wrote one. Parsed values don't have this problem because the mask handler already accumulates prefixes for them. This is another case of #403 and #237

With this PR, if the previous declaration is the same property with an identical value, we merge the vendor prefixes into it instead of pushing a duplicate. The same merge is applied to generated @supports color fallbacks, which are registered per authored declaration and would otherwise keep the duplicate.

Only adjacent declarations are merged, since mask is a shorthand and merging across an intervening declaration (e.g. a mask-image that the later mask resets) could change the cascade. Merging also requires exact token equality, so identical duplicate declarations collapse too, which is cascade-neutral.

The logic lives in a small helper (UnparsedProperty::merge_prefixes_into_last) since the same bug exists in other handlers (transition, animation, transform, border-radius) — those are left for follow-ups to keep this focused, as is the mask-border/-webkit-mask-box-image pair, which needs property mapping rather than prefix normalization. The approach follows #850, which fixed the same kind of duplicate inside transition-property values, and doesn't overlap with #1259/#1284, which fix a sibling bug in the parsed backdrop-filter path.

Related to #969