XrAnchors - Availability flag

[Maksims]

For the consistency across the API and avoiding slight edge cases that can be present between different WebXR Specs implementations by vendors, this PR introduces available flag like on other WebXR APIs, that if false will not try to update underlying anchors data and other functions will not be available.

This also fixes #5843 although should mention that Emulator is misbehaving because of incorrect specs implementation (polyfill). But this PR overcomes that nevertheless.

New APIs:

// pc.XrAnchors
anchors.available // true of false depending on availability of the API and if session was started with Anchors feature requested
anchors.on('available', () => { }); // when session starts, if anchors are supported and were requested for the session, this will trigger
anchors.on('unavailable', () => { }); // if anchors were available and session ended, this will trigger

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