Rename goto_split top/bottom directions to up/down. (#3427)

Renames the top/bottom directions of `goto_split` to up/down. I have
tested this on linux (nixos) but given that `goto_split` is broken on
linux anyway (#2866) there's not a whole lot to test.

I have no way to build on macOS so I can't verify that I've changed
everything correctly for that.

Closes #3237

src/apprt/action.zig

@@ -332,9 +332,9 @@ pub const GotoSplit = enum(c_int) {
     previous,
     next,
 
-    top,
+    up,
     left,
-    bottom,
+    down,
     right,
 };
 

src/apprt/gtk/Split.zig

@@ -339,7 +339,7 @@ pub fn directionMap(self: *const Split, from: Side) DirectionMap {
             // This behavior matches the behavior of macOS at the time of writing
             // this. There is an open issue (#524) to make this depend on the
             // actual physical location of the current split.
-            result.put(.top, prev.surface);
+            result.put(.up, prev.surface);
             result.put(.left, prev.surface);
         }
     }
@@ -347,7 +347,7 @@ pub fn directionMap(self: *const Split, from: Side) DirectionMap {
     if (self.directionNext(from)) |next| {
         result.put(.next, next.surface);
         if (!next.wrapped) {
-            result.put(.bottom, next.surface);
+            result.put(.down, next.surface);
             result.put(.right, next.surface);
         }
     }