feeature(bt): Add "unpair combo" on startup.

* Especially for splits, we need the ability to
  unpair all paired devices as  sledgehammer
  if we need to "reset things", and doing so via
  keymaps isn't suitable.
* Allows shields to define a collection of key
  positions that if all held 2 seconds after
  startup, will unpair all existing pairs for the
  device.

app/boards/shields/corne/corne.dtsi

@@ -58,6 +58,10 @@ RC(2,1) RC(2,2) RC(2,3) RC(2,4) RC(2,5)  RC(2,6) RC(2,7) RC(2,8) RC(2,9) RC(2,10
 		
 	};
 
+	bt_unpair_combo: bt_unpair_combo {
+		compatible = "zmk,bt-unpair-combo";
+	};
+
 	// TODO: per-key RGB node(s)?
 };