Add option to set split direction (#16345)

This adds an option to set the split direction for both the horizontal
splits, and the vertical splits.

A couple of things to look for when reviewing:

* The `derive` keywords on the Enums were copy pasted, no clue what they
should be
* Tried adding tests for this, but got stuck.

Co-authored with @Tobbe

Fixes: https://github.com/zed-industries/zed/issues/11342

assets/settings/default.json

@@ -69,6 +69,10 @@
   // The factor to grow the active pane by. Defaults to 1.0
   // which gives the same size as all other panes.
   "active_pane_magnification": 1.0,
+  // The direction that you want to split panes horizontally. Defaults to "up"
+  "pane_split_direction_horizontal": "up",
+  // The direction that you want to split panes horizontally. Defaults to "left"
+  "pane_split_direction_vertical": "left",
   // Centered layout related settings.
   "centered_layout": {
     // The relative width of the left padding of the central pane from the