API for clientside router (#8571)

* refactored CSR into goto() function

* first refectoring for router API

* added test

* added comments to fixture

* rename + preliminary changeset

* changeset: now 'minor' and featuring Mathew's example from the docs

* moved for simpler diff

* update after #8617

* fixed ts-errors

* more comprehensible handling of intra-page state

* sync with main

* synch from next_tm

.changeset/fresh-pots-draw.md

@@ -0,0 +1,17 @@
+---
+'astro': minor
+---
+
+View transitions can now be triggered from JavaScript!
+
+Import the client-side router from "astro:transitions/client" and enjoy your new remote control for navigation:
+
+```js
+import { navigate } from 'astro:transitions/client';
+
+// Navigate to the selected option automatically.
+document.querySelector('select').onchange = (ev) => {
+  let href = ev.target.value;
+  navigate(href);
+};
+```

packages/astro/client.d.ts

@@ -120,6 +120,13 @@ declare module 'astro:transitions' {
 	export const ViewTransitions: ViewTransitionsModule['default'];
 }
 
+declare module 'astro:transitions/client' {
+	type TransitionRouterModule = typeof import('./dist/transitions/router.js');
+	export const supportsViewTransitions: TransitionRouterModule['supportsViewTransitions'];
+	export const transitionEnabledOnThisPage: TransitionRouterModule['transitionEnabledOnThisPage'];
+	export const navigate: TransitionRouterModule['navigate'];
+}
+
 declare module 'astro:middleware' {
 	export * from 'astro/middleware/namespace';
 }