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/caps_word.dtsi

@@ -4,11 +4,15 @@
  * SPDX-License-Identifier: MIT
  */
 
+#include <dt-bindings/zmk/behaviors.h>
 #include <dt-bindings/zmk/keys.h>
 
 / {
     behaviors {
-        /omit-if-no-ref/ caps_word: caps_word {
+#if ZMK_BEHAVIOR_OMIT(CAPS_WORD)
+        /omit-if-no-ref/
+#endif
+        caps_word: caps_word {
             compatible = "zmk,behavior-caps-word";
             #binding-cells = <0>;
             continue-list = <UNDERSCORE BACKSPACE DELETE>;