Fix transitions that use var()

[geforcefan]

Transitions never start when the transition property uses a variable:

body { --fast: 0.3s cubic-out; }
button { transition: background-color var(--fast); }

The same rule with a plain 0.3s cubic-out animates, the var() version does not.

TransitionPropertyChanges reads the local transition property before variables are resolved, so the value is still a var expression instead of a TRANSITIONLIST and the function returns early. Resolve the variables there the same way other properties do, then run the existing type check.