[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.js

@@ -94,6 +94,13 @@ Attacker.prototype.mutate_sword_attack_damage = function(value) {
   return true;
 };
 
+/**
+ * @returns {string}
+ */
+Attacker.getFullyQualifiedName = function() {
+  return 'Attacker';
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  */
@@ -171,6 +178,13 @@ Rapunzel.prototype.mutate_hair_length = function(value) {
   return true;
 };
 
+/**
+ * @returns {string}
+ */
+Rapunzel.getFullyQualifiedName = function() {
+  return 'Rapunzel';
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  * @param {number} hair_length
@@ -224,6 +238,13 @@ BookReader.prototype.mutate_books_read = function(value) {
   return true;
 };
 
+/**
+ * @returns {string}
+ */
+BookReader.getFullyQualifiedName = function() {
+  return 'BookReader';
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  * @param {number} books_read
@@ -348,6 +369,13 @@ Movie.prototype.charactersLength = function() {
   return offset ? this.bb.__vector_len(this.bb_pos + offset) : 0;
 };
 
+/**
+ * @returns {string}
+ */
+Movie.getFullyQualifiedName = function() {
+  return 'Movie';
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  */