Add SmartNewline command with tests and editor integration

- Introduced `CommandId::SmartNewline` for auto-indented newlines, enhancing text editing workflows.
- Added `cmd_smart_newline` to implement indentation-aware newline logic.
- Integrated SmartNewline with keymaps, mouse/keyboard input handlers, and terminal/editor commands.
- Wrote comprehensive tests in `test_smart_newline.cc` to validate behavior for spaces, tabs, and no-indentation cases.
- Updated `Command.h` and `CMakeLists.txt` to register and build the new command.

CMakeLists.txt

@@ -336,6 +336,7 @@ if (BUILD_TESTS)
             tests/test_visual_line_mode.cc
             tests/test_benchmarks.cc
             tests/test_migration_coverage.cc
+            tests/test_smart_newline.cc
 
             # minimal engine sources required by Buffer
             PieceTable.cc