nifs attribute completion

[MarkoMin]

Description

Adds completion for -nifs attribute officially supported from OTP25

EDIT: dont merge yet, I have to fix completion inside "-nifs" attribute to behave similarly as inside "-export"

[MarkoMin]

I pushed the solution, but tests are missing ATM. It's not as elegant as I hoped, but did my best. It doesn't exclude functions already defined as -nifs. I tried to do it, but ended up too deeply in els code. If you have an idea how do to this more elegant, I'm all ears.

Will add tests when I find more time

[plux]

I tried out the feature locally and it works fine.

Seems like there some formatting issues.
Please run rebar3 fmt and commit the changes.

If you find time to write some tests that would also be awesome :)