Implement hitstop shake (#42)

Reviewed-on: #42
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/fighter/state/hitstun.rs

@@ -95,4 +95,13 @@ impl FighterStateImpl for HitstunState {
             }
         }
     }
+    
+    fn is_visually_grounded<'a>(
+        &'a self,
+    ) -> bool {
+        match self.hitstop_kind {
+            HitstopStateKind::Grounded(_) => true,
+            HitstopStateKind::Airborne => false,
+        }
+    }
 }