More precise Object.entries type declarations

[ethanresnick]

Uses the new index type queries and indexed access types from #11929 to get better type inference for Object.entries.

Before:

Object.entries({a: 1, b: 2}) // [string, number][]
Object.entries({a: true, b: 2}) // [string, number | boolean][]
Object.entries("foo") // [string, any][]

After:

Object.entries({a: 1, b: 2}) // ['a' | 'b', number][]
Object.entries({a: true, b: 2}) // ['a' | 'b', number | boolean][]
Object.entries("foo") // [string, any][]

One edge case is Object.entries({}). Before, that was inferred as [string, {}][]; with this change it'd be [never, any][]. That might be a problem, although I don't think it is, since never is assignable to string.

[msftclas]

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[mhegazy]

since you made that change, object.keys seems like another place we can make a similar change.