New user option 'grep-use-headings'

New user option 'grep-use-headings'

* lisp/progmodes/grep.el (grep-heading-regexp): New user option.
(grep-heading): New face (bug#59888).
(grep--heading-format, grep--heading-state, grep--heading-filter):
Filter function for grep processes and supporting variables.
(grep-use-headings): New user option.
(grep-mode): Use the above, if applicable.