[Javascript] Fix syntax error for signed enum (#5503)

* wrap quotes to enum name map to prevent syntax errorn when enum value is negative

* Add a test that covers signed enum case

tests/MyGame/Example/Race.php

@@ -0,0 +1,27 @@
+<?php
+// automatically generated by the FlatBuffers compiler, do not modify
+
+namespace MyGame\Example;
+
+class Race
+{
+    const None = -1;
+    const Human = 0;
+    const Dwarf = 1;
+    const Elf = 2;
+
+    private static $names = array(
+        Race::None=>"None",
+        Race::Human=>"Human",
+        Race::Dwarf=>"Dwarf",
+        Race::Elf=>"Elf",
+    );
+
+    public static function Name($e)
+    {
+        if (!isset(self::$names[$e])) {
+            throw new \Exception();
+        }
+        return self::$names[$e];
+    }
+}