fix(types): correct type of Datastore.KEY (#459)

`entity.KEY_SYMBOL` is defined as a `unique symbol` and without explicit reference back to the original type, the `unique symbol` flows into a regular `symbol`, which loses some of its semantic meaning.

This guarantees that `Datastore.KEY` and `entity.KEY_SYMBOL` (and `new Datastore().KEY`) are truly equivalent within the type system.

Author: ckknight

handwritten/datastore/src/index.ts

@@ -576,8 +576,8 @@ class Datastore extends DatastoreRequest {
    * @name Datastore#KEY
    * @type {symbol}
    */
-  static KEY = entity.KEY_SYMBOL;
-  KEY = Datastore.KEY;
+  static KEY: typeof entity.KEY_SYMBOL = entity.KEY_SYMBOL;
+  KEY: typeof entity.KEY_SYMBOL = Datastore.KEY;
 
   /**
    * This is one of three values which may be returned from