Intent to prototype: Transferable streams

[Kagami Rosylight]

Summary: We want to let stream objects be transferable via postMessage().

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1659025

Specification: https://streams.spec.whatwg.org/#rs-transfer, https://streams.spec.whatwg.org/#ws-transfer, https://streams.spec.whatwg.org/#ts-transfer

Standards Body: WHATWG

Platform coverage: All

Preference: dom.streams.transferable.enabled (will be initially enabled only on Nightly)

Other browsers: Shipped on Chrome in 2020, no signal from Safari.

web-platform-tests: https://wpt.live/streams/transferable/