Extension blocks are not foldable

[envoidia]

Version Used: 5.3.0-2.25603.1

Expected Behavior: Extension blocks can be folded just like any other block/scope

Actual Behavior: They can't be

Additionally, when an extension block is in sticky scroll, it is (presumably unintentionally) always the top line of sticky scroll, replacing the class it's inside

[envoidia]

The PR that added sticky scroll to them (#81667) also added folding, according to its name.

Oddly, I get sticky scroll, but not folding, which shouldn't be possible if they were both added in the same commit, which is what confused me. Will revisit later.