refactor: improve code quality with component extraction and type safety

[meetp90]

This commit includes four refactoring improvements to enhance code maintainability and type safety:

1. Extract TabListItem component

   • Moved inline list item JSX to separate TabListItem component
   • Improves code organization and testability
   • File: packages/docusaurus-theme-classic/src/theme/Tabs/TabListItem.tsx

2. Make TOC highlight selectors configurable

   • Added headingAnchorClassName and navbarSelector to TOCHighlightConfig
   • Removes hardcoded class dependencies for better theme customization
   • File: packages/docusaurus-theme-common/src/hooks/useTOCHighlight.ts

3. Improve NavbarItem types with discriminated unions

   • Replaced loose type definition with proper discriminated unions
   • Better type safety, autocomplete, and compile-time error detection
   • File: packages/docusaurus-theme-common/src/utils/useThemeConfig.ts

4. Use path.posix.relative() in sidebar generator

   • Replaced manual string manipulation with Node.js built-in utility
   • Better maintainability and correctness
   • File: packages/docusaurus-plugin-content-docs/src/sidebars/generator.ts

All changes are backward compatible and all existing tests pass.

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Test Plan

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

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