Auto merge of #443 - CYBAI:getpromiseishandled, r=jdm
Introduce GetPromiseIsHandled
Expose `GetPromiseIsHandled` API to `rust-mozjs`.
This would be used in servo/servo#20755 for the step 4.1.1 for the spec of [notify about rejected promises](https://html.spec.whatwg.org/multipage/webappapis.html#notify-about-rejected-promises).
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