bug#59888: [PATCH] Add 'grep-heading-mode'

[Mattias Engdegård]

No argument about the idea (good to me) or implementation but minor points 
regarding the regexp:

> +          (group-n 1 (+? any))

Very much prefer `nonl` or `not-newline` to `any`, which is only there for 
compatibility (shouldn't have been included in the first place).

> +          (any "\0-:="))

Is the range (ASCII 00..1A) intended here, or should that be a literal hyphen?
If the former, it should probably be made more explicit since that range 
includes all sorts of numbers, symbols and control chars and it kind of looks 
like it may be a mistake.