Interoperability with Django and Jinja using __html__() protocol

[collinanderson]

Would be nice to be able to use fasthtml within an existing Django project, as an alternative to Django's html templates.

Django and Jinga already interop with each other using a special __html__() method for communicating whether a string has already been html-escaped.

https://jinja.palletsprojects.com/en/3.1.x/templates/#jinja-filters.escape