feat: Add the ability to keep/omit behaviors for a given build.

* Use defines to keep either all behaviors with omits, or selective behaviors with
  explicit kept behavior, before including `behavior.dtsi` in keymaps.
* Default ZMK_BEHAVIORS_KEEP_ALL when building with the studio RPC
  endpoint snippet.

app/dts/behaviors/toggle_layer.dtsi

@@ -4,9 +4,14 @@
  * SPDX-License-Identifier: MIT
  */
 
+#include <dt-bindings/zmk/behaviors.h>
+
 / {
     behaviors {
-        /omit-if-no-ref/ tog: toggle_layer {
+#if ZMK_BEHAVIOR_OMIT(TOG)
+        /omit-if-no-ref/
+#endif
+        tog: toggle_layer {
             compatible = "zmk,behavior-toggle-layer";
             #binding-cells = <1>;
             display-name = "Toggle Layer";