feat: Add MultiSelect widget and update imports

- Implement MultiSelect widget inheriting from Checkbox
- Update import order in __init__.py for consistency
- Prepare groundwork for multi-select dropdown functionality

lib/chatmark/__init__.py

@@ -1,6 +1,6 @@
 from .form import Form
 from .step import Step
-from .widgets import Button, Checkbox, Radio, TextEditor, MultiSelect
+from .widgets import Button, Checkbox, MultiSelect, Radio, TextEditor
 
 __all__ = [
     "Checkbox",

lib/chatmark/widgets.py

@@ -162,6 +162,7 @@ class Checkbox(Widget):
 
         self._selections = selections
 
+
 class MultiSelect(Checkbox):
     """
     ChatMark syntax: