feat: Add TableOfContents component and fix Slug field button styling

[ainsleyclark]

Summary

This PR adds a new TableOfContents component to the sveltekit-helper package with scrollspy functionality, and fixes a styling issue in the Payload helper's Slug field component.

Key Changes

TableOfContents Component (New)

• Added new TableOfContents.svelte component with the following features:
  • Auto-discovery: Automatically discovers headings from DOM using configurable CSS selectors
  • Scrollspy: Tracks scroll position and highlights the active section in the TOC
  • Flexible configuration: Supports both auto-generated items from DOM and manually provided items via props
  • Data attributes: Uses data-sidebar-content and data-sidebar-selector attributes for DOM discovery
  • Customizable: Props for heading, border display, content selector, heading selector, and scroll offset
  • Styling: Includes responsive design with optional left border on tablet and above
• Exported new component from packages/sveltekit-helper/src/components/index.ts

Slug Field Component Fix

• Fixed build error in packages/payload-helper/src/fields/Slug/Component.tsx by:
  • Removing unsupported style prop from Payload's Button component
  • Wrapping the button in a <span> element to apply the styling instead

Implementation Details

• The TableOfContents component uses Svelte 5's $state and $props runes for reactivity
• Scrollspy detection includes a bottom threshold to ensure the last section is highlighted when scrolled to the bottom
• The component is designed to work alongside richtext/content areas with heading elements that have id attributes
• Priority order for heading selector: explicit prop > data-sidebar-selector attribute > default h2

https://claude.ai/code/session_01Qrcz9fPxJBQk2GD1G3ZAug

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.26%. Comparing base (7f6b060) to head (9218ceb).
⚠️ Report is 514 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #429      +/-   ##
==========================================
+ Coverage   64.59%   70.26%   +5.67%     
==========================================
  Files         154      187      +33     
  Lines        6064     7439    +1375     
==========================================
+ Hits         3917     5227    +1310     
+ Misses       2064     2012      -52     
- Partials       83      200     +117     

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