Gate autoimports behind the experimental completions flag

[SomeoneToIgnore]

Part of #6612

Adds a possibility to disable autoimports:

and other experimental completions we might want to add later.

[kjeremy]

Code LGTM.

Since it's labeled as experimental we might want to make the default false.

[kjeremy]

Actually I'll pull it in since it's not a regression. We can make it false in a follow-up PR if people think that's a good move.

[kjeremy]

bors r+

[bors]

Build succeeded:

• Rust (ubuntu-latest)
• Rust (windows-latest)
• Rust (macos-latest)
• TypeScript (windows-latest)
• TypeScript (ubuntu-latest)

[SomeoneToIgnore]

I've made true by default to be consistent with what we do with experimental diagnostics (same) and because not that many people had complained about the autoimports (yet?)

[flodiebold]

TBH, I would just add a completion.autoimport.enable setting. I wouldn't call the autoimport completions experimental, and I don't think there will be so many "experimental" completion types that we need to group them 🤔

[extremegf]

and because not that many people had complained about the autoimports (yet?)

I'm a simple Rust developer and from my perspective buggy auto-complete with imports is better than bug-free limited auto-complete.