feat(feedback): Add onClose callback to showReportDialog (#9433)

[arya-s]

Adds an onClose callback to showReportDialog.

The callback is invoked when a __sentry_reportdialog_closed__ MessageEvent is received. This is sent from the error page embed via window.postMessage and listened to in the sdk.

Please have a look at the accompanying PRs

• for the backend: feat(feedback): Sends MessageEvent when error page embed is closed sentry#59885
• for the docs: feat(feedback): Add onClose description to user-feedback sentry-docs#8484

And here is a sample project that uses onClose to color the background green again after closing the widget:
https://github.com/arya-s/sentry-onclose-report-dialog

Caveats

• Any __sentry_reportdialog_closed__ message will call onClose, so if users use more than one widget per page it could wrongly trigger onClose. I figured the widget takes over the entire page and having more than one wouldn't make much sense, but if this is an issue I could generate a key in the sdk and pass it along to the backend and specifically listen for it back, please advise.
• The jest test for when onClose throws, prevents jest from failing when encountering the uncaught exception when firing off the MessageEvent. This seems a bit hackish, and I had to dig into jest-environment-jsdom's source to figure out how to prevent the test from failing.
  Curiously, I couldn't reproduce this on codesandbox: https://codesandbox.io/s/distracted-dubinsky-7x59dy?file=/src/index.test.js

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[AbhiPrasad]

Any sentry_reportdialog_closed message will call onClose, so if users use more than one widget per page it could wrongly trigger onClose. I figured the widget takes over the entire page and having more than one wouldn't make much sense

There should only be one widget on the page at a time, this implementation is fine.