generate size-prefixed monster binary as well

tests/monster_test_ps.fbs

@@ -0,0 +1,10 @@
+// similar schema, except to be parsed with --prefix-size
+
+include "monster_test.fbs";
+
+namespace MyGame.Example;
+
+root_type Monster;
+
+file_identifier "MONS";
+file_extension "mops";