[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/MyGame/Example/NestedStruct.py

@@ -9,6 +9,10 @@ np = import_numpy()
 class NestedStruct(object):
     __slots__ = ['_tab']
 
+    @classmethod
+    def SizeOf(cls):
+        return 32
+
     # NestedStruct
     def Init(self, buf, pos):
         self._tab = flatbuffers.table.Table(buf, pos)