Filter the "On This Page" TOC based on a selected language selection.

[alistairmatthews]

This functionality is intended to help site visitors see only content for their preferred language in the AppHost: C# or TypeScript.

This PR:

• Implements a selector for the language.
• Filters the content of an article for that selection.
• Also filters the headings displayed in the "On This Page" TOC. This is something the existing pivot selector doesn't do.

To test:

• pnpm run dev
• Navigate to /get-started/prerequisites (Filter is currently only implemented on this page)
• Under Choose your AppHost language select your preference.
• Content changes both in the article body and in the One The Page TOC.