[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/union_vector/union_vector_generated.js

@@ -185,6 +185,13 @@ Rapunzel.getFullyQualifiedName = function() {
   return 'Rapunzel';
 }
 
+/**
+ * @returns {number}
+ */
+Rapunzel.sizeOf = function() {
+  return 4;
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  * @param {number} hair_length
@@ -245,6 +252,13 @@ BookReader.getFullyQualifiedName = function() {
   return 'BookReader';
 }
 
+/**
+ * @returns {number}
+ */
+BookReader.sizeOf = function() {
+  return 4;
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  * @param {number} books_read