[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.ts

@@ -376,6 +376,13 @@ mutate_b(value:number):boolean {
   return true;
 };
 
+/**
+ * @returns number
+ */
+static sizeOf():number {
+  return 4;
+}
+
 /**
  * @param flatbuffers.Builder builder
  * @param number a
@@ -668,6 +675,13 @@ test3(obj?:MyGame.Example.Test):MyGame.Example.Test|null {
   return (obj || new MyGame.Example.Test()).__init(this.bb_pos + 26, this.bb!);
 };
 
+/**
+ * @returns number
+ */
+static sizeOf():number {
+  return 32;
+}
+
 /**
  * @param flatbuffers.Builder builder
  * @param number x
@@ -811,6 +825,13 @@ mutate_distance(value:number):boolean {
   return true;
 };
 
+/**
+ * @returns number
+ */
+static sizeOf():number {
+  return 8;
+}
+
 /**
  * @param flatbuffers.Builder builder
  * @param number id