Intent to ship: Temporal

[Daniel Minor]

As of Firefox 139, I intend to enable Temporal by default on all platforms. It is currently available in Nightly builds, enabled by default and controlled by the `javascript.options.experimental.temporal` preference. Temporal is a (much) better version of Date. Our implementation was contributed by a volunteer, André Bargull. Temporal is a large proposal and this was an enormous amount of work. Thanks Anba :)

The proposal is currently at Stage 3 of the TC39 process. V8's implementation is in progress (https://issues.chromium.org/issues/42201538) and so is JavaScriptCore's (https://bugs.webkit.org/show_bug.cgi?id=223166).

Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=1912511

Standard: https://github.com/tc39/proposal-temporal