emacs: add a shortcut for ac-merlin-locate but with merlin-locate-preference flipped

[kit-ty-kate]

By default C-c C-l (ac-merlin-locate) follows the merlin-locate-preference setting (defaults to the implementation file). However i feel like it would be handy to have another shortcut but where this preference is flipped: defaulting to the interface file.

[voodoos]

We had this discussion recently with @xvw who is working on a new, eglot-based, mode for Emacs (which does not yet support the various kind of locate) and we decided to provide multiple commands instead of a configuration option. I agree it feels like a better way to select the target of locate since it is a switch that is often needed.

We probably won't have the time to update the current Emacs mode ourselves, but will be happy to review a PR adding such a command.