Run init.ds from engineInit once every other system is ready

assets/init.ds is compiled and packed as init.dsb like any other script, and
engineInit() runs it as the very last init step - after scriptInit() and
sceneInit(), right before the initial scene is set - so it's free to use any
engine system (display, audio, save, rpg, ui, scenes, ...) without having to
special-case startup ordering. A syntax error or runtime failure in init.ds
is fatal, same as every other errorChain()'d init step.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

assets/init.ds

@@ -0,0 +1 @@
+// Entry point, run once by the engine after every other system is ready.