Automatically generate table of contents in text

[ed-kung]

Description

Closes #2208

Writing {:toc} on its own line will automatically render a table of contents as a bulleted list, using the headings (#, ##, etc) contained in the post.

Screenshots

auto-toc.mov

Additional Context

n/a

Checklist

Are your changes backwards compatible? Please answer below:

yes

On a scale of 1-10 how well and how have you QA'd this change and any features it might affect? Please answer below:

8. If another post had {:toc} somewhere it could render in unexpected ways.

The behavior of links that start with a hash (i.e. #section-1) has also changed, but I can't see why anyone would've wanted such links to open in a new tab.

For frontend changes: Tested on mobile, light and dark mode? Please answer below:

n/a

Did you introduce any new environment variables? If so, call them out explicitly here:

no