[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.cs

@@ -0,0 +1,17 @@
+// <auto-generated>
+//  automatically generated by the FlatBuffers compiler, do not modify
+// </auto-generated>
+
+namespace MyGame.Example
+{
+
+public enum Race : sbyte
+{
+  None = -1,
+  Human = 0,
+  Dwarf = 1,
+  Elf = 2,
+};
+
+
+}