Intent to prototype: endpoint-inclusive commitStyles

[Kanaru Sato]

Summary:
Make `Animation.commitStyles` use the final value of the animation interval even if a corresponding fill mode is not set. This allows authors to more easily persist animation effects without using fill modes which are generally discouraged.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1947061
Specification: https://github.com/w3c/csswg-drafts/pull/11881
Standards Body: CSS WG. See issue 5394: https://github.com/w3c/csswg-drafts/issues/5394
Platform coverage: All platforms
Preference: dom.animations.commit-styles-endpoint-inclusive
DevTools bug: n/a
Link to standards-positions discussion: The proposed change (which we are driving) is intended to be included in Web Animations 1 which we ship.
Other browsers: We are the first to attempt to implement this. In the past Blink and WebKit have indicated their willingness to adopt this change. Once we have determined the change is Web-compatible, we will approach them again.
web-platform-tests: https://phabricator.services.mozilla.com/D238391

Best,
canalun