Rework Markdown parsing

[ginnyTheCat]

Description

Simplify markdown parsing to allow for new features and better spec compliance.

• Spoilers
  • ||spoiler|| syntax
  • new ||spoiler||(reason) syntax
    • Show reason when displaying spoilers #370
  • plain text fallback
• Inline math using $1+1
  • respecting rules outlined in Do not treat everything between two $ as TeX math #709
• Display math using $$1+1$$
  • without newlines $$1+1$$ (new)
  • with newlines $$\n1+1\n$$
• Plain text rendering

Fixes #709
Fixes #193
Fixes #749 (replicate element behavior)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Preview: https://6302170673d245070a1fa41c--pr-cinny.netlify.app
⚠️ Exercise caution. Use test accounts. ⚠️

[ajbura]

Is it ready for review?

[ginnyTheCat]

i think it's ready.
the plaintext rendering is not completed yet as making ||test|| being sent as [spoiler](mxc://spoiler/content-here) would make it annoying and confusing to edit messages with a spoiler as this is what would show up in the edit field. I'm thinking about how to mitigate that but atm the only possibility i see is parsing the html version.
but we can merge the pr and add that later.

[ginnyTheCat]

Sorry for messing up the commits a bit. Accidentally did a rebase which clashed with your merge. Hope it's ok now

[ajbura]

Can we replace \n with <br /> instead of applying client specific css for newlines?

[ginnyTheCat]

this is to space <p> tags further apart to make them appear as actual paragraphs (as shown in the screenshots of #193). this does not relate to \n or <br> afaik.

[ajbura]

Looks great. Thank you :).