embark-export: Support grep-use-headings

[minad]

I've just migrated to Emacs 30. There is a new option grep-use-headings which we could also support in Embark grep export.

[oantolin]

Sounds good, but I have zero clue what that option does. 😁

[minad]

From the NEWS file:

*** New user option 'grep-use-headings'.
When non-nil, the output of Grep is split into sections, one for each
file, instead of having file names prefixed to each line.  It is
equivalent to the "--heading" option of some tools such as 'git grep'
and 'rg'.  The headings are displayed using the new 'grep-heading'
face.

The output looks a little bit like consult-grep in Vertico with the separator lines per file. I had somewhat hoped that grep-use-headings would just work automagically with the buffers export by Embark, but it does not seem like it.