Add support for unset CSS keyword to getComputedStyle()

[jsnajdr]

Add support for the CSS-wide unset keyword when computing styles in getComputedStyle(). An unset value removes any previously set values for the element and forces it to be inherited or have initial value.

A companion PR to #3478, they should probably get merged together. The inspiration for this PR was seeing unset being handled in @testing-library/user-event's closestPointerEventsDeclaration helper.

[domenic]

Looks great. I'm unclear why this should or should not be merged together with #3478; would something bad happen if we merged and released this before doing #3478 and #3482? I'll hold off for now, but let me know.

[jsnajdr]

This is fully independent from the other two PRs (#3478 and #3482) and can be merged separately. It will only cause a merge conflict with #3482, because both PR touch the same code, the loop that goes through matching style rules and applies their properties, but that's fine.