Add is:built-in-kotlin to search autocomplete and advanced filters #9670

https://pub.dev/packages?q=is%3Abuilt-in-kotlin

is:built-in-kotlin is a pana tag (added in dart-lang/pana#1592) marking Android plugins that build Kotlin with AGP's built-in Kotlin instead of the Kotlin Gradle Plugin. Searching for it works, but typing is: in the search bar never suggests it, so it's hard to discover. is:swiftpm-plugin and is:wasm-ready are both suggested.

Also add a "Built-in Kotlin" checkbox to the Advanced filters, matching SwiftPM/WASM. That touches pkg/index.dart, _pub_shared/lib/search/tags.dart, search_form.dart, and the search_page.html / pkg_index_page.html goldens.
https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin/for-plugin-authors

Related: dart-lang/pana#1589.