[JS] Add getFullyQualifiedName() (#6119)

This optionally generates a static `getFullyQualifiedName()` function to get
the fully-qualified name of a type in JavaScript and TypeScript in a similar
fashion to the C++ codegen.

tests/namespace_test/namespace_test1_generated.ts

@@ -72,6 +72,13 @@ mutate_foo(value:number):boolean {
   return true;
 };
 
+/**
+ * @returns string
+ */
+static getFullyQualifiedName():string {
+  return 'NamespaceA.NamespaceB.TableInNestedNS';
+}
+
 /**
  * @param flatbuffers.Builder builder
  */
@@ -190,6 +197,13 @@ mutate_b(value:number):boolean {
   return true;
 };
 
+/**
+ * @returns string
+ */
+static getFullyQualifiedName():string {
+  return 'NamespaceA.NamespaceB.StructInNestedNS';
+}
+
 /**
  * @param flatbuffers.Builder builder
  * @param number a