refactor(boards): Move Corneish Zen to ZMK variant by default

Make the standard Corneish Zen board ID be `corneish_zen_left/nrf52840/zmk` or
`corneish_zen_left//zmk` by shorthand and for right as well.

app/boards/lowprokb/corneish_zen/board.yml

@@ -3,6 +3,8 @@ boards:
     vendor: lowprokb
     socs:
       - name: nrf52840
+        variants:
+          - name: zmk
     revision:
       format: major.minor.patch
       default: 2.0.0
@@ -14,6 +16,8 @@ boards:
     vendor: lowprokb
     socs:
       - name: nrf52840
+        variants:
+          - name: zmk
     revision:
       format: major.minor.patch
       default: 2.0.0

app/boards/lowprokb/corneish_zen/corneish_zen.zmk.yml

@@ -15,5 +15,5 @@ revisions:
   - 2.0.0
   - 1.0.0
 siblings:
-  - corneish_zen_left
-  - corneish_zen_right
+  - corneish_zen_left//zmk
+  - corneish_zen_right//zmk