[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.go

@@ -0,0 +1,35 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package Example
+
+import "strconv"
+
+type Race int8
+
+const (
+	RaceNone  Race = -1
+	RaceHuman Race = 0
+	RaceDwarf Race = 1
+	RaceElf   Race = 2
+)
+
+var EnumNamesRace = map[Race]string{
+	RaceNone:  "None",
+	RaceHuman: "Human",
+	RaceDwarf: "Dwarf",
+	RaceElf:   "Elf",
+}
+
+var EnumValuesRace = map[string]Race{
+	"None":  RaceNone,
+	"Human": RaceHuman,
+	"Dwarf": RaceDwarf,
+	"Elf":   RaceElf,
+}
+
+func (v Race) String() string {
+	if s, ok := EnumNamesRace[v]; ok {
+		return s
+	}
+	return "Race(" + strconv.FormatInt(int64(v), 10) + ")"
+}