Markdown alerts

[vanillajonathan]

Add support for markdown alerts.

See: https://github.com/orgs/community/discussions/16925

[toolstack]

Seems unlikely due to how Folio manages the editor, but I'll keep this open for the future.

[vanillajonathan]

Folio already supports blockquotes and this is just variant of blockquotes but with different color (and a icon and name of the alert type).

[toolstack]

All those things are the difficult parts to add 😉