Re: scratch/backend-completion 9a62da21c2 1/2: Integrate Stefan suggestions but rename it to "external-completion.el"

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> For prefix completion, it works great.
>> For completion styles like `flex`, it's rarely able to complete anything.
> Alright, that helps, thanks. But what use can you imagine try-completion
> having with an external tool like LSP or my everything.exe client which
> does regex-style?  Or some other external tool you can think of...

There's one I can think of: SLY, because there external tool's
completion code can be tweaked to fit the needs of Emacs :-)

For most others it's probably difficult, indeed, which is why
I suggested to make it optional (unless you know how the tool
does its matching in which case you may be able to re-use
`completion-pcm--merge-try` or to implement something similar).

> Else, I think we end up contacting the external tool one too many times for
> the same PATTERN. That's at least with the popular completion front-ends
> such as fido/vertico who probably go for the 'allc' immediately.

The default UI uses either tryc (when you hit TAB) or allc (when you
ask the see *Completions*) not both.


        Stefan