[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.lua

@@ -0,0 +1,12 @@
+-- automatically generated by the FlatBuffers compiler, do not modify
+
+-- namespace: Example
+
+local Race = {
+    None = -1,
+    Human = 0,
+    Dwarf = 1,
+    Elf = 2,
+}
+
+return Race -- return the module