Consider unifying specifications

[yoavweiss]

We have a few specifications that may benefit from unification:

• Beacon could incorporate the PendingBeacon proposal if/when it gains consensus.
• PaintTiming can incorporate LCP as well as ElementTiming (once it's adopted)
• requestIdleCallback can be expanded to a "Scheduling" spec that will include also isInputPending and postTask as well as future scheduling APIs. Alternatively, those could be incorporated directly into HTML.

I'd love thoughts and feedback on the above.

[yoavweiss]

Also ResourceTiming and NavigationTiming may be a good candidate for unification.

[yoavweiss]

Added this as planned work in https://github.com/w3c/web-performance/commit/22551cab75187ceda548ab884e513c605cf39956