This attempts to use a global copy event listener instead of
triggering the copy handler on Ctrl+C, as this is more robust way to
use browser APIs, including not intefering if end users choose to
remap copying keys on their keyboard.
This has various beneficial side effects, including copying from other
Markdown elements like the preview or drafts UI using the same code as
copying from the message feed.
Fixes: #33949.
Note that the puppeteer tests only tested
copying whereas the node tests only tested
pasting, which is why the puppeteer tests
for pasting and node tests for copying are
absent after this split.