Intent to prototype: HTMLMediaElement.captureStream()

[Alastor Wu]

Summary:
`HTMLMediaElement.captureStream()` is a standards-based API that exposes a `MediaStream` capturing the output of an `<audio>` or `<video>` element.

We propose implementing a spec-compliant `captureStream()` in Gecko to replace the legacy `mozCaptureStream()`. Our plan is to keep both APIs available initially, and then gradually deprecate the legacy API in the future to improve web compatibility and align with other browsers.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1541471

Specification: https://w3c.github.io/mediacapture-fromelement/#html-media-element-media-capture-extensions

Preference: media.captureStream.enabled

Platform coverage: All

Other browsers:  https://caniuse.com/wf-capture-stream-audio-video
Blink (Chromium): Supported

WebKit (Safari): Not supported