feat: customizable Scrollbars

[ndonkoHenri]

Fix #5912

Test Code

# Test code for the review of this PR

Summary by Sourcery

Introduce a configurable Scrollbar API for scrollable controls and migrate existing scroll behavior to use it.

New Features:

• Add a Scrollbar dataclass and ScrollbarOrientation enum to configure scrollbar appearance and placement in scrollable controls and pages.
• Allow ScrollableControl and Page.scroll to accept either legacy ScrollMode presets or detailed Scrollbar configurations.
• Provide interactive Python examples demonstrating Scrollbar customization and ScrollbarOrientation behavior.

Enhancements:

• Refactor Flutter-side scrollbar handling into a ScrollbarConfiguration utility, translating legacy ScrollMode values and custom configuration maps into Flutter Scrollbar properties.
• Update scrollable control rendering to always use the unified ScrollbarConfiguration, simplifying auto-scroll logic and removing the old ScrollMode.none path.
• Expose the new scrollbar utilities in the Dart public API and Python package exports, and extend documentation to cover the new types.

[cloudflare-workers-and-pages]

Deploying flet-examples with    Cloudflare Pages

Latest commit:	0c409c8
Status:	✅  Deploy successful!
Preview URL:	https://e295041c.flet-examples.pages.dev
Branch Preview URL:	https://scroll.flet-examples.pages.dev

View logs

[cloudflare-workers-and-pages]

Deploying flet-docs with    Cloudflare Pages

Latest commit:	0c409c8
Status:	✅  Deploy successful!
Preview URL:	https://2b3e5963.flet-docs.pages.dev
Branch Preview URL:	https://scroll.flet-docs.pages.dev

View logs