Object.keys: more precise types

Applying the same logic used on Object.entries in the prior commit and
in microsoft#12207

Author: ethanresnick

src/lib/es5.d.ts

@@ -210,6 +210,8 @@ interface ObjectConstructor {
       * Returns the names of the enumerable properties and methods of an object.
       * @param o Object that contains the properties and methods. This can be an object that you created or an existing Document Object Model (DOM) object.
       */
+    keys<T>(o: Array<T>): string[];
+    keys<T extends { [key: string]: any }>(o: T): (keyof T & string)[];
     keys(o: any): string[];
 }