feat: add in keyword completions

[zardoy]

Don't see a reason too add a setting to disable them. There are awesome!

Feature Description

Typescript docs says that in operator is usually used for object type narrowing. Example from the test:

Numbers on right side of the completions means type index, starting from actual position in the union (starting from 1).
When specific key is selected, completion widget displays the type of key in each type of the union.

[zardoy]

Just noticed

const a: { a: boolean, b: string } | { a: number, c: number } | string = {} as any
if ('/*|*/' in a) {}

• in test: uncorrect results
• in vscode: weird crash

blocked, since i need to investigate both cases...