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/boards/native_posix_64.overlay

@@ -1,4 +1,4 @@
-#include <behaviors.dtsi>
+
 #include <dt-bindings/zmk/keys.h>
 #include <dt-bindings/zmk/kscan_mock.h>
 

app/boards/shields/minivan_studio_tester/minivan_studio_tester.keymap

@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: MIT
  */
 
+#define ZMK_BEHAVIORS_KEEP_ALL
+
 #include <behaviors.dtsi>
 #include <dt-bindings/zmk/keys.h>