[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/union_vector/union_vector_generated.ts

@@ -107,6 +107,13 @@ mutate_sword_attack_damage(value:number):boolean {
   return true;
 };
 
+/**
+ * @returns string
+ */
+static getFullyQualifiedName():string {
+  return 'Attacker';
+}
+
 /**
  * @param flatbuffers.Builder builder
  */
@@ -207,6 +214,13 @@ mutate_hair_length(value:number):boolean {
   return true;
 };
 
+/**
+ * @returns string
+ */
+static getFullyQualifiedName():string {
+  return 'Rapunzel';
+}
+
 /**
  * @param flatbuffers.Builder builder
  * @param number hair_length
@@ -289,6 +303,13 @@ mutate_books_read(value:number):boolean {
   return true;
 };
 
+/**
+ * @returns string
+ */
+static getFullyQualifiedName():string {
+  return 'BookReader';
+}
+
 /**
  * @param flatbuffers.Builder builder
  * @param number books_read
@@ -442,6 +463,13 @@ charactersLength():number {
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 };
 
+/**
+ * @returns string
+ */
+static getFullyQualifiedName():string {
+  return 'Movie';
+}
+
 /**
  * @param flatbuffers.Builder builder
  */