Fix DeepKeys to handle arrays properly

[mimbrown]

The types for nested arrays in the DeepKeys type seem definitely wrong to me. The type properly handles tuples, but not arrays of arbitrary length. For example, given this type:

DeepKeys<{
  foo: { bar: string }[];
}>

This currently resolves to "foo" | "foo.bar", which is unexpected. If we change the type to be a tuple (i.e. { bar: string }[] --> [{ bar: string }]), then we correctly get "foo" | "foo.0.bar".

This change updates the handling for arbitrary length arrays so that the above type will resolve to:

"foo" | `foo.${number}` | `foo.${number}.bar`

This puts the onus on the user to make sure the number is not out of range, but that seems reasonable to me.