dcs: sort of implement DCS passthrough

We now store the passthrough characters in a buffer, and call
dcs_passthrough() on unhook.

However, dcs_passthrough() doesn't do anything.

dcs.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include <stdbool.h>
+#include "terminal.h"
+
+void dcs_passthrough(struct terminal *term);
+bool dcs_ensure_size(struct terminal *term, size_t required_size);