🐍 Add parser support for additional markup features

[fwkoch]

This PR includes some work from SciPy sprints to support a wider variety of markdown syntaxes:

• roles from shortcodes, e.g. {{< role value >}}
• labels at the end of headings/images/etc, e.g. {#my-label}
• classes on spans, e.g. [content]{.python}
• colon fences can be used to create divs with classes

[changeset-bot]

⚠️ No Changeset found

Latest commit: f1107bb

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[choldgraf]

Being able to add classes to spans and divs is really nice, especially if we can get custom CSS rules supported:

e.g. either:

• Add support for loading user-defined CSS stylesheet myst-theme#321
• Or an issue that lets people define custom CSS in page metadata

[agoose77]

@rowanc1 we spoke about this at SciPy and I recall talking about which side of the markup the {} should go for spans. I think we also thought about whether roles should change too.

Can you remember any of the conclusions? It was frenetic time...

[rowanc1]

I don't think this is ever intended to merge as is, just highlighting a few of the experiments we did at scipy!