It turns out all we need to do is to set lineNumbers: true in 2 places in the MediaWiki-extensions-CodeMirror codebase. This is how it would look like in the wikitext editor:

This is how it would look like in VisualEditor, without any extra styling. Note the visualeditor-newwikitext BetaFeature must be enabled to be able to switch to this mode.

For comparison, this is how it looks in the code editor that can be enabled on Lua, JavaScript, and CSS pages:

Remarks:

• When dealing with code, tools show error messages like "syntax error in line 42". But this never happens when dealing with wikitext. In other words: The actual numbers are pretty much meaningless. However, …
• … the numbers indicate where the next line starts. This could as well be done with a symbol, but that could be confused with bullet points. Increasing numbers probably tell a more convincing story.
• … the numbers allow to see how long an article is. Before, the scaling of the scroll bar was the only indicator – and sometimes not even that, depending on the browser.
• … it's easier to navigate back to a position. Even if I don't remember the exact number, I usually have a vague idea of how big the number was.

Oops started writing the description before I saw your comment @thiemowmde - very exciting! I'm glad it's so straightforward. It doesn't even look too bad in VE, I would start by simply removing the background and the darker line on the right side so they are just floating light gray numbers and see how that looks. If it could get a bit of padding on the right side (8px?), then even better.

Also, I agree with your remarks analysis of why this would be helpful. I think the main benefit is seeing newlines in an easily recognizable and clear way, the rest are the side benefits that we can predict now, there may be more. It might also help people who are collaborating on the same template code as well.

I suspect all styling can easily done with CSS. Having different styles for the different situations is also possible.

@thiemowmde Can this be easily enabled on the test instance? Would be nice to be able to try it out :)

Change 618737 had a related patch set uploaded (by Thiemo Kreuz (WMDE); owner: Thiemo Kreuz (WMDE)):
[mediawiki/extensions/CodeMirror@master] [DNM] Experimentally enable line numbering

https://gerrit.wikimedia.org/r/618737

By hot-patching the code, maybe? I uploaded a patch that can do this. @awight, are you able to apply this to our test machine?

In T259252#6365538, @thiemowmde wrote:

By hot-patching the code, maybe? I uploaded a patch that can do this. @awight, are you able to apply this to our test machine?

Deployed!

Looks great! And I think already demonstrates how it can be useful.