Make `Function` alias to `{ [[call]]: any }`

[goodmind]

Proposal

Make Function alias to { [[call]]: any } instead of any

Use case

It would error if you provide non-callable things and also seems more safe, instead of outright any

[kevinbarabash]

Is { [[call]]: any } equivalent to { (...args: any[]): any }?

I didn't know about [[call]], are there any docs for it? I can't seem to find docs for callable types either.

[goodmind]

There's PR for call property #7790 docs, not sure about equivalency

[SamChou19815]

We now updated the docs to recommend other safer patterns: https://flow.org/en/docs/types/functions/#any-function