Add the ability to change the code block language in rich text mode

[b-kelly]

Reported on Meta.SE.

This was mentioned in another answer I believe, but I would like to add some mock UX for an idea of how it can work and look.

I just added an arrow next to the language to make it look like a selector.

When clicking on the language, a dialogue would appear with possible selection. For example, a list similar to the following.

✓ auto
-------
c#
css
html
javascript
plaintext
python

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Another idea would be to show a dialogue similar to the image's details dialogue. Although this can be annoying and would much prefer the option above.