Add support for copying code snippets

[kingjuno]

What does this PR do?
This adds a feature to copy code snippets as mentioned in #1123

Work in progress
Tested?

• Manually
• Existing tests (adapted, if necessary)
• New tests added (for any new behavior)
• Passed linting & tests (each commit)

[neiljp]

@kingjuno Thanks for the work on this, I fell short of time to review sooner. Would you like to take this forward?

[neiljp]

I just rebased this to remove conflicts and get it working minimally.

This would fix most of #1123.

For anyone taking this forward, this would benefit from fixing the linting issues, splitting the commits, and adding tests for separate parts that are built into the final version. That includes in the latter commits first just showing the code snippet in the message-info popup first, before then adding the action.

Please attribute kingjuno and this PR if you take this forward, but open your own based on this PR. Once a PR is merged we will then close related PRs, and possibly the original issue.

[neiljp]

#1353 is the current successor to this PR.

[zulipbot]

Heads up @kingjuno, we just merged some commits that conflict with the changes you made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/main branch and resolve your pull request's merge conflicts accordingly.