make auto-update of nextls configurable

[dch]

nextls is automatically fetched for more common OS & platforms, but for other
platforms it should be possible to disable the auto-update, or at least
handle it more gracefully than just bailing out.

Currently I'm just building nextls locally in a repo and starting it listening
on the appropriate port as required.

What's a reasonable approach here? I see a few options:

• provide a way to skip auto-update but still have nextls usable
• handle download failure more gracefully with:
  • bail with a reasonable error message if not MacOS or Linux
  • ditto for other architectures
  • check if curl is available
  • handle failure of curl itself

[mhanberg]

If you set the cmd field to the location of your locally built exe, it will disable auto update and still run it through stdio as usual.

[mhanberg]

Also do you mean auto update or the auto install?

Auto update is done by next ls (updates itself in place) and auto install is when the nvim plugin downloads it if it can't find one and cmd isn't set

[mhanberg]

Also what platform are you on?