Fill the outer edges of generated levels

To avoid the character being able to go OOB.

This is something we had in the Rust version but I hadn't ported over yet

src/Xanthous/Generators/CaveAutomata.hs

@@ -98,6 +98,7 @@ generate' params dims = do
   let steps' = params ^. steps
   when (steps' > 0)
    $ for_ [0 .. pred steps'] . const $ stepAutomata cells dims params
+  lift $ fillOuterEdgesM cells
   pure cells
 
 stepAutomata :: forall s g. MCells s -> Dimensions -> Params -> CellM g s ()