[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.js

@@ -205,6 +205,13 @@ NamespaceA.NamespaceB.StructInNestedNS.getFullyQualifiedName = function() {
   return 'NamespaceA.NamespaceB.StructInNestedNS';
 }
 
+/**
+ * @returns {number}
+ */
+NamespaceA.NamespaceB.StructInNestedNS.sizeOf = function() {
+  return 8;
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  * @param {number} a