Decrease priority of compilation jobs to increase UX responsiveness.

[mgajda]

HIE is expected to respond to events quickly, just like its ancestor is expected to be UI process.
However, the compilations are generally batch jobs that can take longer time to complete,
and should not delay UI.

I suggest we use System.Process.nice to decrease priority of any compilation job we call.
That assures our interactivity is unhindered when we open a big project, and start compiling it with all numCapabilities.

This continues discussion from here:

haskell/haskell-language-server#3528

[mgajda]

@michaelpj Is that what you suggested in haskell/haskell-language-server#3528?

[fendor]

Yeah, I think that is sensible. I'd merge a PR for it.