Word completion in text modes

[Eshel Yaron]

Hi Emacs,

I wonder how people here feel about updating `text-mode` to have it (and
its derivatives) use `completion-at-point` for word completion instead
of `ispell-complete-word`.  That'd be accompanied by a new
`ispell-completion-at-point` function that `text-mode` adds to
`completion-at-point-functions` so word completion still works OOTB,
just using a different interface.

I can see several advantages to making such a change:

1. Word completion would benefit from UI customizations and
   other enhancements that users have in place for `completion-at-point`.
2. `completion-preview-mode` and any other feature that leverages
   `completion-at-point-functions` would work for word completion as well.
3. Emacs would be slightly more consistent and simple all in all.

The main downside of this idea is that people who prefer the interface
that `ispell-complete-word` provides would need to rebind it to `C-M-i`
or some other key themselves.  Since this command has been around for a
very long time, I can imagine that some users may have grown fond of it.

So I have a WIP patch that adds `ispell-completion-at-point` (similar to
the one proposed in Bug#52743, but slightly enhanced) and updates
`text-mode` to use it.  If that sounds sensible and people are not that
attached to the `ispell-complete-word` interface for word completion,
I'll add some documentation updates and submit that patch for review.


Thanks,

Eshel