Add support for Mercury (https://mercurylang.org) in 'etags'

Add support for Mercury (https://mercurylang.org) in 'etags'

Tag declarations starting lines with ':-'.
By default, all declarations are tagged.  Optionally, first
predicate or functions in clauses can be tagged as in Prolog
support using '--declarations'.  (Bug#47408).
* lib-src/etags.c (test_objc_is_mercury, Mercury_functions)
(mercury_skip_comment,  mercury_decl, mercury_pr):
Implement Mercury support.  As Mercury and Objective-C have
the same file extension .m, a heuristic test tries to detect
the language.

* doc/man/etags.1: Document the change.  Add Mercury-specific
 behavior for '--declarations'.  This option tags first
predicates or functions in clauses in addition to declarations.