fix(templates): Add siblings to split shield

Updated the .zmk.yml file for a split shield to include the siblings
list.

Fixes #17

Author: joelspadin

zmk/templates/shield/split/${id}.zmk.yml

@@ -3,3 +3,7 @@
 requires:
   - ${interconnect}
 % endif
+
+siblings:
+  - ${id}_left
+  - ${id}_right