add stronger typings for Object.keys

[Arcath]

Object.keys is currently typed as outputting string[] which is technically correct but causes issues when you forxample want to use the key with the original object if that object does not have an index signature.

const foo = {
   a: {
    bar: 'something'
  },
  b: {
    bar: 'else'
  }
}

Object.keys(foo).map((key) => {
  return foo[key].bar // Not valid because `foo` has no index signature
})

This is a bit annoying as key can only be a valid index of foo but TS won't let you use it as such.

Using this PR the above code works as key has the type keyof foo which is usable to index foo.

I have to apologise as the tests aren't passing at the moment. I'm struggling to see where they are failing and could do with some help. I also can't sign in to cla.opensource.microsoft.com to sign the CLA.

[cpplearner]

See #12253 (comment).

[Arcath]

Thanks, I should have looked into it first. Stick with my function for now.

[RyanCavanaugh]

https://stackoverflow.com/questions/55012174/why-doesnt-object-keys-return-a-keyof-type-in-typescript