6631: Gate autoimports begind experimental completions flag r=kjeremy a=SomeoneToIgnore
Part of https://github.com/rust-analyzer/rust-analyzer/issues/6612
Adds a possibility to disable autoimports:
<img width="598" alt="image" src="https://user-images.githubusercontent.com/
2690773/
100156673-
f8037f80-2eb1-11eb-8e74-
59ebe4260ba3.png">
and other experimental completions we might want to add later.
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>