Move IsMovingVertically to IMove

- avoids an Aircraft trait look-up in #11886
- potentially useful for future additional locomotors (jumpjet), custom locomotors in external mods, possibly future updates to default ground locomotor (Mobile) with regard to behavior on slopes

OpenRA.Mods.Common/Traits/Mobile.cs

@@ -327,6 +327,7 @@ namespace OpenRA.Mods.Common.Traits
 		readonly Actor self;
 		readonly Lazy<IEnumerable<int>> speedModifiers;
 		public bool IsMoving { get; set; }
+		public bool IsMovingVertically { get { return false; } set { } }
 
 		int facing;
 		CPos fromCell, toCell;