Extending syntax highlighter languages broken in v7

[shilman]

Originally posted by @i-am-the-slime in #7847 (comment)

@shilman, I believe this is also broken in v6 (#16080 (comment)). This may be slightly off-topic, but it would also be nice to disable syntax highlighting completely (#19702 (discussion)). Let me know if you'd like me to create a separate issue for the discussion referenced.

[ndelangen]

@vanessayuenn should we mark this as a nice-to-have in the 7.0 burndown?

[j3rem1e]

Is there a workaround for this issue ?

I used to add highlighting for java and Svelte in sb V6 but it doesn't work in v7

[JReinhold]

@ndelangen wasn't this fixed in #23166?