Use a map instead of a hook to jump to grep result

[lobre]

This has been discussed a little bit on Discord, but I think we do not need a hook for the <ret> mapping in the grep module.

I guess this was something that was necessary in the past, but I am not sure that we need it anymore.

Also, it prevents from overriding the <ret> key easily at the user level with:

map global normal <ret> "..."

[krobelus]

I was also surprised why we use a hook here instead of a mapping.

Turns out there is a behavioral difference; consider

map global normal <c-j> <ret>

This mapping will fire NormalKey <ret> hooks, thus triggering the existing mapping.
It would not trigger the proposed mapping.

I wonder if we should move away from relying on this subtle difference.
One could use execute-keys -with-maps instead.

There is a related problem; this remapping is currently broken in that it doesn't add to prompt history:

map global prompt <c-j> %{<a-semicolon>:execute-keys -with-hooks -with-maps <lt>ret><ret>}

luckily I don't need maps/hooks in prompt mode so I can use the simple remapping.

There is the argument that if we used NormalKey hooks everywhere,
then the simple remapping (map global normal <c-j> <ret>) would work, no need for execute-keys.
But of course it's surprising that normal mode key-aliases are mapped differently than for other modes.
I'm not yet sure what we should do.
Of course we should be less inconsistent in the end.

---

Either way we still want to remove the mapping when the filetype changes.
A naive

hook -once -always window WinSetOption filetype=.* %{ unmap buffer <ret> :jump<ret> }

will have problems; particularly if we switch from filetype=grep to another
filetype that adds this mapping via a filetype hook.
Since hooks run in definition order, the unmap hook will run last and remove
the wrong mapping.

We can solve that by making the maps unique (map buffer <ret> ':jump<ret> # grep').

---

I wonder if instead of mapping <ret> everywhere, we should define a buffer-scoped option (or alias).
That might make it easier to map keys like <c-j> to do the same as <ret> but it kinda goes against our conventions (<ret> is just that, there is no other name), so I'd not go there.