[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.py

@@ -0,0 +1,10 @@
+# automatically generated by the FlatBuffers compiler, do not modify
+
+# namespace: Example
+
+class Race(object):
+    None_ = -1
+    Human = 0
+    Dwarf = 1
+    Elf = 2
+