Implement user-defined collapsible section state

[XanNuri]

Discussed in #565

^{Originally posted by aolko January 5, 2023}
How do i make certain sections of the page collapsed by default?

Would like to submit this as a feature request! To allow per section to be collapsed by default

[alistair3149]

This is quite complicated since skins don't typically have much access to the content of the page.
Implementing a feature like that would require putting something into the parser, which is something that I am not comfortable to do at the moment especially with the on-going Parsoid work.

This would be revisited in Citizen 3.X after MW 1.43 drops, as the section parsing logic is built into Parsoid (https://phabricator.wikimedia.org/T363934).