Update execCommand use in context-menu-copy-link-with-types

[rebloor]

context-menu-copy-link-with-types uses a content script because when the example was written execCommand didn't work in background pages; now they do so the example should be updated to call execCommand from the background page instead of using a content script.

Note, before updating check whether Clipboard.write() is available in Firefox, as this would be the preferred method given that execCommand is deprecated.

MDN URL: https://developer.mozilla.org/en-US/docs/Web/API/Clipboard/write
MDN URL: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons

[caugner]

Clipboard.write() is still only partially supported in Firefox and behind a feature flag according to the BCD table, so I added the "on hold" label for now.

@rebloor Would Clipboard.writeText() not work for this example?

[rebloor]

@rebloor Would Clipboard.writeText() not work for this example?

Haven't looked into that. Will do, when this gets to the top of the list.