add Function.prototype.toString result for callable proxies

test/built-ins/Function/prototype/toString/proxy.js

+// Copyright (C) 2018 Michael Ficarra. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-function.prototype.tostring
+description: Function.prototype.toString on a proxied function
+info: |
+  Function.prototype.toString accepts any callable, including proxied
+  functions, and produces a NativeFunction
+includes: [nativeFunctionMatcher.js]
+---*/
+
+const f = new Proxy(function(){}, {});
+assertNativeFunction(f);
+
+const g = new Proxy(f, { apply() {} });
+assertNativeFunction(g);