9$: warn on 'keyof T[]' in type

[strager]

In a TypeScript type, keyof T[] is the same as number (I think). The user likely meant (keyof T)[]. We should emit a warning, telling them to write either number or add parentheses.

Inspired by this article: https://www.totaltypescript.com/array-types-in-typescript

Using keyof with T[] can lead to unexpected results.

const result: keyof Person[] = ["id", "name"];

[Jetoson]

@strager May I work on this?