Add inclusion support for simple block tags

[Jake Howard]

Django's template system allows creating tags which include other templates:

@register.inclusion_tag("results.html")
def show_results(poll):
    choices = poll.choice_set.all()
    return {"choices": choices}

However, with the addition of simple_block_tag in #35535, it would be nice to combine the 2 into a inclusion_block_tag. This creates a form of reusable component for Django template, combining context transformation, rendering and internal content. There are a number of libraries in the Django ecosystem which achieve this (eg slippers), so there's clearly an appetite for this functionality.

@register.inclusion_block_tag("results.html")
def show_results(content, poll):
    choices = poll.choice_set.all()
    return {"content": content, "choices": choices}

{% show_results poll %}
  This content gets captured too
{% endmysimpletag %}

The API matches the existing inclusion_tag, but with an additional required content argument containing the (rendered) template content.

[Jake Howard]

An open question I have about this approach: Should content be implicitly passed through to the template, or require passing in via the context? It feels like a case of "explicit > implicit", however it's also a fairly easy foot-gun and confusion for developers.