Updates markdown-it to v13.0.1
Noteworthy changes:
* `markdownit()` is now available on `globalThis` instead of `window`.
* The `text_collapse` rule was renamed to `fragments_join` which affected the `bbcode-inline` implementation.
* The `linkify` rule was added to the `inline` chain which affected the handling of the `[url]` BBCode. If available, our implementation reuses `link_open` and `link_close` tokens created by linkify in order to prevent duplicate links.
* The rendered HTML for code changed slightly. There's now a linebreak before the `</code>` tag. The tests were adjusted accordingly.
