WebXR manual room capture

[Maksims]

With Quest 3 and similar devices, environment is captured at the special time instead of real-time like for Android devices.
So the extension to the WebXR Plane Detection Specs provides a new method for XRSession to initiate manual room capture.

This can be useful when developer want to use AR with Plane Detection or Real-World Meshes, and needs an up to date information, or user might have not pre-captured their environment at all. So developer can provide an option to trigger manual capture right from the AR session.
Our current API integration design takes asynchronous nature of real-world objects like planes into the account, so changes in underlying data will be reflected immediately without a need to refresh a page.

New APIs:

// pc.XrManager
xr.initiateRoomCapture((err) => { })
// Start manual room capture.
// If such feature is not supported, error with according message will be provided in callback.
// If capture was a successful, then callback will fire without an error.

I confirm I have read the contributing guidelines and signed the Contributor License Agreement.