feat: provide migration function (#11334)

Provides a migration function, exported as `migrate` from `svelte/compiler`, which tries its best to automatically migrate towards runes, render tags (instead of slots) and event attributes (instead of event handlers)

The preview REPL was updated with a migrate button so people can try it out in the playground.

closes #9239

Author: dummdidumm

.changeset/orange-zoos-heal.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+feat: provide migration helper

packages/svelte/src/compiler/index.js

@@ -190,5 +190,5 @@ export function walk() {
 }
 
 export { CompileError } from './errors.js';
-
 export { VERSION } from '../version.js';
+export { migrate } from './migrate/index.js';