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

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 same file extension
.m,  a heuristic test tries to detect the language.
If this test fails, --language=mercury should be used.
* 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.