Publish an initial HTML Fullscreen monkeypatch spec

[michaelwasserman]

This task tracks the work item of publishing an initial monkeypatch spec for the HTML Fullscreen API.

Chromium shipped an initial implementation in M127/M128, but the pertinent Fullscreen API issue and pr may take a while to settle.

Discussion during the W3C Second Screen WG's TPAC 2024 meeting suggested codifying spec changes in a monkeypatch for now.