add visibleRanges support for FileRenderer

[BrianHung]

Description

This PR adds collapse/expand functionality to the File component, mirroring the existing diff hunks UI/UX.

New FileRenderer API:

• visibleRanges: LineRange[] - specify which line ranges to show
• expansionLineCount: number - lines to reveal per expand click
• hunkSeparators: HunkSeparators - separator style for collapsed regions

I've also added a demo to apps/demo/index.html

Motivation & Context

I would like to have same UI/UX for collapsed or hidden lines, but for FileRenderer.

Screenshot

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)
• Documentation update

Checklist

• I have read the
  contributing guidelines
• My code follows the code style of the project (bun run lint)
• My code is formatted properly (bun run format)
• I have updated the documentation accordingly (if applicable)
• I have added tests to cover my changes (if applicable)
• All new and existing tests pass (bun run diffs:test)

[vercel]

@BrianHung is attempting to deploy a commit to the Pierre Computer Company Team on Vercel.

A member of the Team first needs to authorize it.