Remove packaging.cmake, deprecate test_undo setup, and add new testing infrastructure.

- Delete `packaging.cmake` to streamline build system.
- Deprecate `test_undo` in CMake setup; condition builds on `BUILD_TESTS`.
- Introduce `TestFrontend`, `TestRenderer`, and `TestInputHandler` for structured testing.
- Update `GUIInputHandler` and `Command` for enhanced buffer save handling and overwrite confirmation.
- Enhance kill ring operations and new prompt workflows in `Editor`.

TerminalInputHandler.cc

@@ -85,15 +85,8 @@ map_key_to_command(const int ch,
 	// ESC as cancel of prefix; many terminals send meta sequences as ESC+...
 	if (ch == 27) {
 		// ESC
-		k_prefix = false;
-		esc_meta = true; // next key will be considered meta-modified
-		// Cancel any universal argument collection
-		uarg_active     = false;
-		uarg_collecting = false;
-		uarg_negative   = false;
-		uarg_had_digits = false;
-		uarg_value      = 0;
-		uarg_text.clear();
+		k_prefix       = false;
+		esc_meta       = true; // next key will be considered meta-modified
 		out.hasCommand = false; // no command yet
 		return true;
 	}