feat: add Scrollable component

[ahmedkhan1]

All Submissions:

• I installed pre-commit prior to committing my changes.
• My commit messages conform to conventional commits.
• My PR title conforms to conventional commits.
• I linked to the relevant issue.

Changes to / New Features:

• I included docs for my feature.
• I wrote tests for my feature.

Description of changes

Adds a public solara.Scrollable layout component backed by the existing Div API. It applies a configurable maximum height and vertical auto-scrolling while preserving custom classes and styles.

The documentation includes a runnable long-Markdown example, and the unit test covers the public export, rendered container, child content, classes, and composed styles.

Closes #70

Validation

• pytest tests/unit/scrollable_test.py -q (1 passed)
• pytest tests/unit -q (552 passed, 37 skipped)
• pre-commit run --all-files (all non-type hooks passed; both full mypy hooks hit the same five existing Windows platform errors in untouched files)
• changed-file mypy with default and mypy-v3.ini configurations (passed)