Currently a condition can only be a regexp or a major-mode symbol
but there's no real reason to not allow any predicate as a
condition.
* doc/misc/autotype.texi: Document the new condition type in
'auto-insert-alist'.
* etc/NEWS: Announce the new condition type in 'auto-insert-alist'.
* lisp/autoinsert.el (auto-insert-alist): Add the new condition
type to the variable documentation.
(auto-insert): Change the way the condition is matched to allow for
custom predicates. (Bug#79178)
