Support DCG and SSU rules in sweeprolog-insert-next-clause

Support DCG and SSU rules in sweeprolog-insert-next-clause

* sweeprolog.el (sweeprolog-definition-at-point): also return kind of
neck.
(sweeprolog-maybe-insert-next-clause): pass it to...
(sweeprolog-insert-next-clause): new argument NECK used instead of
hardcoded ":-", use "Body" for clause body instead of "_".
(sweeprolog-identifier-at-point): handle raw meta goals.
* sweeprolog-tests.el: add tests for sweeprolog-insert-term-dwim
inserting clauses with different neck kinds.