[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/namespace_test/namespace_test1_generated.ts

@@ -204,6 +204,13 @@ static getFullyQualifiedName():string {
   return 'NamespaceA.NamespaceB.StructInNestedNS';
 }
 
+/**
+ * @returns number
+ */
+static sizeOf():number {
+  return 8;
+}
+
 /**
  * @param flatbuffers.Builder builder
  * @param number a