feat: defined state and state machine concepts

modules/wave_survival/state.cpp

@@ -0,0 +1,16 @@
+#include "state.h"
+
+void State::_bind_methods() {
+}
+
+String State::get_next_state() const {
+	return this->get_class();
+}
+
+void State::set_state_machine(StateMachine *machine) {
+	this->state_machine = machine;
+}
+
+StateMachine *State::get_state_machine() const {
+	return this->state_machine;
+}