Speed up 'maven' compilation error message regexp

Speed up 'maven' compilation error message regexp

Anchor the regexp at line-start to prevent quadratic behaviour when
it doesn't match (bug#39595).  It's unclear whether the type tag, like
[ERROR], is always present; we keep it optional just in case.

* lisp/progmodes/compile.el (compilation-error-regexp-alist-alist):
Rewrite 'maven' regexp, using rx for clarity.
* etc/compilation.txt (maven): More examples.
* test/lisp/progmodes/compile-tests.el
(compile-tests--test-regexps-data): No leading spaces; they seems to
stem from a misunderstanding in bug#11517.