[go_router_builder] Add support for `TypedStatefulShellBranch`'s `preload`#8587

auto-submit[bot] merged 3 commits intoflutter:mainflutter/packages:mainfrom

ValentinVignal:go-router-builder/support-preloadValentinVignal/flutter__packages:go-router-builder/support-preloadCopy head branch name to clipboard