fix: Make CopyToClipboard popup optimistic

[TrevorBurnham]

Description

This PR is a fix for the longstanding visual glitch when clicking CopyToClipboard: Instead of briefly showing the popover in a "Pending" state, it uses the Permissions API ahead of time to determine whether the clipboard API is likely to succeed, then optimistically shows the popover in the "Success" or "Error" state depending on that value.

This has the advantage that the user gets immediate feedback that will be correct virtually every time, rather than having to wait for the async Clipboard API to resolve. The downside is that edge cases are possible: A user could disable clipboard permissions after the CopyToClipboard component renders, for instance, in which case the popover would incorrectly show a success message for a moment before showing an error message. I think this edge case behavior is acceptable, given the tradeoffs imposed on us by the async Clipboard API.

This is a simpler alternative to the previously-suggested fix at #3443.

How has this been tested?

I've run the dev server and clicked around in the CopyToClipboard demos. It's a much, much smoother experience with the immediate feedback this PR provides.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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