feat(conditional-layers): Implement feature

This is a generalization of the existing concept of tri-layer support
that's already well known. Essentially, a conditional-layer
configuration activates a particular layer (the then-layer) when one or
more other layers (the if-layers) are activated.

This is commonly used on ortho keyboards to activate a third "adjust"
layer while the primary two layers ("lower" and "raise") are active.

app/dts/bindings/zmk,conditional-layers.yaml

@@ -0,0 +1,17 @@
+# Copyright (c) 2021 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: Conditional layers allow layer combinations to trigger additional layers
+
+compatible: "zmk,conditional-layers"
+
+child-binding:
+  description: "Single conditional layer that activates then-layer when if-layers are active"
+
+  properties:
+    if-layers:
+      type: array
+      required: true
+    then-layer:
+      type: int
+      required: true