Fix visibility of Tab's close button when onClosed is null

[diluculo]

Added missing isClosable property to Tab widget to match WinUI 3's TabViewItem.IsClosable implementation.

The property defaults to true. When isClosable is false:

• Close button is not shown
• Keyboard shortcuts (Ctrl+W, Ctrl+F4) are disabled

• I have updated CHANGELOG.md with my changes
• I have run "dart format ." on the project
• I have added/updated relevant documentation

[bdlukaa]

We use a different pattern in Flutter. If the onClose callback is null, it is automatically marked as disabled, therefore there is no need for another property.

[diluculo]

Thank you for the feedback about onClosed.

I was actually trying to find a way to hide the close button in certain tabs. Looking at the current code, I noticed that the close button is shown even when onClosed is null.

To address this, I reworked it to simplify the implementation by adding a null check for onClosed.

As shown in the image, the first tab has onClosed = null. With the updated implementation, its close button is now properly hidden, while other tabs show their close buttons as expected. This matches the behavior of the TabView control in WinUI 3, which hides the close button when IsClosable is false.

[bdlukaa]

Can you add it to the changelog?

[diluculo]

@bdlukaa, Done! I added the fix to the changelog and also squashed the commits into a single one.