feat: Added virtual scroll component and sample implementation

[rkaraivanov]

Description

Expose a new virtual scroll component for horizontal/vertical scrolling.

Motivation / Context

1. Self-contained viewport — The component is the scrollable element. igxForOf requires the developer to manually wire a separate scroll container, a VirtualHelperComponent/HVirtualHelperComponent DOM helper element, and pass igxForContainerSize explicitly. That's 3–4 pieces of setup vs. one tag.
2. Signals-first reactive model.
3. View pooling — Active views are kept in a pool and detached/reattached instead of destroyed/recreated on every scroll. igxForOf uses ViewContainerRef.move() but always keeps a fixed chunk alive, paying create/destroy cost when chunk size changes.
4. Clean ChangeDetectionStrategy.OnPush throughout the implementation.
5. Better DX for remote/infinite scrolling.
6. No auxiliary helper components in the DOM.
7. Typed template context.

Type of Change (check all that apply):

• Bug fix
• New functionality
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (no functional changes)
• Documentation
• Demos
• CI/CD
• Tests
• Changelog
• Skills/Agents

Component(s) / Area(s) Affected:

How Has This Been Tested?

• Unit tests
• Manual testing
• Automated e2e tests

Test Configuration:

• Angular version:
• Browser(s):
• OS:

Screenshots / Recordings

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them
• Accessibility (ARIA, keyboard navigation, focus management) has been verified