markdown: Add quoted emoticon parser function. (Issue #17789)

[schultzh]

#17789

In this approach, any emoticon found in "fenced code" is not converted into an emoji. This includes when quoting another user where the initial message includes an emoticon, as to preserve the contents and meaning of the original message.

Testing plan: Manual tests are shown below. If this is an acceptable approach to this issue, testing can be added.

GIFs or screenshots:

[zulipbot]

Heads up @schultzh, we just merged some commits that conflict with the changes your 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.