Trac ticket number

ticket-35535

See also related forum thread: https://forum.djangoproject.com/t/feature-proposal-simple-block-tag/32229

Branch description

Django's simple_tag is difficult to work with if you need to capture more complex content as one of the arguments.

This PR adds the ability to define custom "block" tags, which capture their contents as the content argument.

There are probably some built-in tags which could take advantage of this, but I've left those out-of-scope of this PR. The same is true for extra niceties like a built-in capture tag, passing the un-rendered content into the tag instead or a combination with inclusion_tag etc. See #18343 (comment) for the small diff needed to support inclusion_block_tag in future.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.