[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/NamespaceA/NamespaceB/StructInNestedNS.py

@@ -9,6 +9,10 @@ np = import_numpy()
 class StructInNestedNS(object):
     __slots__ = ['_tab']
 
+    @classmethod
+    def SizeOf(cls):
+        return 8
+
     # StructInNestedNS
     def Init(self, buf, pos):
         self._tab = flatbuffers.table.Table(buf, pos)

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

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