[Python/JS/TS] Codegen SizeOf method for structs (#6136)

* [Python] Codegen SizeOf classmethod for structs

This codegens a `SizeOf()` classmethod for all structs since we can't
determine the size of a FlatBuffer generated struct from Python otherwise.

* [JS/TS] Codegen sizeOf static method for structs

This codegens a `sizeOf()` static method for all structs since we can't
determine the size of a FlatBuffer generated struct from JavaScript or
TypeScript otherwise.

tests/monster_test_generated.js

@@ -342,6 +342,13 @@ MyGame.Example.Test.prototype.mutate_b = function(value) {
   return true;
 };
 
+/**
+ * @returns {number}
+ */
+MyGame.Example.Test.sizeOf = function() {
+  return 4;
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  * @param {number} a
@@ -573,6 +580,13 @@ MyGame.Example.Vec3.prototype.test3 = function(obj) {
   return (obj || new MyGame.Example.Test).__init(this.bb_pos + 26, this.bb);
 };
 
+/**
+ * @returns {number}
+ */
+MyGame.Example.Vec3.sizeOf = function() {
+  return 32;
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  * @param {number} x
@@ -659,6 +673,13 @@ MyGame.Example.Ability.prototype.mutate_distance = function(value) {
   return true;
 };
 
+/**
+ * @returns {number}
+ */
+MyGame.Example.Ability.sizeOf = function() {
+  return 8;
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  * @param {number} id