LibJS: Implement Date.parse()

The spec says Date.parse() should accept at least a simplified form
of ISO 8601, so that's all this implements.

Libraries/LibJS/Runtime/DateConstructor.h

@@ -45,6 +45,7 @@ private:
     virtual bool has_constructor() const override { return true; }
 
     JS_DECLARE_NATIVE_FUNCTION(now);
+    JS_DECLARE_NATIVE_FUNCTION(parse);
     JS_DECLARE_NATIVE_FUNCTION(utc);
 };