Intent to ship: Atomics.pause

[Daniel Minor]

As of Firefox 137, I intend to enable Atomics.pause by default on all platforms. It is currently enabled only in Nightly builds, behind the `javascript.options.experimental.atomics_pause` preference. The Atomics.pause proposal allows using dedicated CPU instructions for microwaits, e.g. when contending for a spinlock, leading to improved power consumption over naive implementations. Our implementation was contributed by a volunteer, André Bargull. Thanks Anba :)

The proposal is currently at Stage 3 of the TC39 process. Both JavaScriptCore (https://webkit.org/blog/16205/release-notes-for-safari-technology-preview-207/) and V8 (https://issues.chromium.org/issues/352359899) have already shipped their implementations.

Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=1937805

Standard: https://github.com/tc39/proposal-atomics-microwait

[Daniel Minor]

I've been a bit sloppy with language here, I meant to say "currently built only in Nightly, and off by default behind the preference", the way I had it phrased originally may have sounded like the pref was already on in Nightly. I'm sorry about any confusion. I'll try to be clearer in the future.