Change on_debugger to register_debugger.

src/bin/rhai-dbg.rs

@@ -220,7 +220,7 @@ fn main() {
     // Hook up debugger
     let lines: Vec<_> = script.trim().split('\n').map(|s| s.to_string()).collect();
 
-    engine.on_debugger(
+    engine.register_debugger(
         // Store the current source in the debugger state
         || "".into(),
         // Main debugging interface