search: add ellipses alias for structural search holes

Created by: rvantonder

I want ... to be an alias for :[_], just easier to type out in a search query, and arguably better readability for just matching things. I implemented it natively in comby, but we still need to transform things here because the regex converter code doesn't recognize ... and will look for that string literally.

Aside: since comby now supports regex, to search literally for ... you can do something like :[~[.]{3}]or whatever.

I'll bundle structural search changelog entries in a separate PR.