Fixes #1884 - Support Spoiler Tags

[ninanator]

WHAT & WHY

Adds support for managing spoiler blocks by:

• Switching to markdown-it.rs, a Rust port of themarkdown-it library used by the UI and, more importantly, supports adding and creating custom plugins.
  • Unfortunately, the library comrak nor another potentially performant library pulldown-cmark do not appear to have any support for plugins so the library switch is necessary.
• Adding a custom spoiler plugin to handle parsing spoiler tags.
• Writing basic tests to ensure that nothing should change due to the library swap. If the library needs to be changed again later, it should help with backwards compatibility checking.

This is my first time writing in Rust, so apologies for any obvious semantic mistakes 🙏

[dessalines]

Thanks for this!

AFAIK this will only affect RSS feeds and some federated objects. The lemmy UIs just receive pure markdown, so they have to have do their own markdown parsers. I still have yet to add some of this stuff into kotlin / jerboa.

[Nutomic]

FYI this will affect RSS feeds, emails, and federation (except with other Lemmy instances in which case the markdown source is used). Nothing else I think.

[ninanator]

@Nutomic @dessalines Thanks for the initial reviews; made some changes based on your feedback. LMK what you think!

[Nutomic]

Thank you!

[dessalines]

Thanks!

[phiresky]

This change is causing issues since markdown-it panics in some cases which breaks feeds and federation. See also #3411 and markdown-it-rust/markdown-it#26

Edit: I also want to mention since this is not mentioned anywhere: This seems to "just" be a subset of fenced_divs from pandoc, which there might be another existing implementation using a different markdown parser for.

[Nutomic]

Looks like the maintainer confirmed that this is a bug in upstream, not related to this change.