Add Push API

[JohnRiv]

As noted in #319, the Push API now actually is supported by all 4 codebases (with Safari recently adding it). This is related to the Notifications API and Service Workers, which are both part of section 3.11 Other web specifications, so that seems to be an appropriate place if/when we decide to include it in WMAS.

However, although the Push API is available in Safari v16.0 and higher, it appears support for it has not yet been added to WPEWebKit, which is the most common WebKit implementation for TVs. So this appears to warrant its inclusion in the Web Media APIs proposed to be supported on all platforms section for the same reasons we include WASM there. Subsection 5.4 Other web specifications of that seems appropriate.

[JohnRiv]

closed via #327