feat: barrier dismissible feature implemented

[Romanbot4]

Add isBarrierDismissible support to modal bottom sheets

• showCustomModalBottomSheet
• showMaterialModalBottomSheet
• showCupertinoModalBottomSheet
• showBarModalBottomSheet

Default value is false to preserve existing behavior

Motivation

Previously, barrier dismissal behavior was implicit or coupled with other properties (such as isDismissible or expand). This change provides explicit and consistent control across Material, Cupertino, and custom modal bottom sheets.

Backward compatibility

This change is backward compatible. Existing behavior remains unchanged unless isBarrierDismissible is explicitly set.

Example

showMaterialModalBottomSheet(
  context: context,
  isBarrierDismissible: true,
  ...
);