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/include/dt-bindings/zmk/behaviors.h

@@ -0,0 +1,9 @@
+/*
+ * Copyright (c) 2024 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#define ZMK_BEHAVIOR_OMIT(_name)                                                                   \
+    !(defined(ZMK_BEHAVIORS_KEEP_##_name) ||                                                       \
+      (defined(ZMK_BEHAVIORS_KEEP_ALL) && !defined(ZMK_BEHAVIORS_OMIT_##_name)))