it's clear i need a rethink of parsing django block tags to nodes,
becuase my mental picture of them differs from the actual use of them.
E.g. I tend to think of them the same as HTML tags as either "void" tags
or ones with end tags and children but not that's not the case at all.
Some have intermediate tags (`{% if %}{% else %}{% endif %}`) which is
not a huge lift to support, but the mind blowing realization was that
it's just convention that the closing tag starts with `end`.

for builtin's, yeah that's it's the case that all of the tags that have
opening and closing tags all close with a matching tag that is the
initial tag with `end` prefixed. but given the flexibility of the django
template engine, a third-party could feasibly use *any* closing tag it
wanted. that makes it very hard to build a structured, full-featured AST
for a Django template.

maybe that's not really needed for an LSP. but i'll need to come up with
*something* eventually.