feat: add SvelteKit adapter

Add framework adapter for SvelteKit with:
- `getApp()` - get bknd app instance
- `serve()` - request handler for hooks.server.ts

Usage in hooks.server.ts:
```typescript
import { serve } from "bknd/adapter/sveltekit";
import config from "../bknd.config";

const bkndHandler = serve(config);

export const handle = async ({ event, resolve }) => {
  if (event.url.pathname.startsWith("/api/")) {
    return bkndHandler(event);
  }
  return resolve(event);
};
```

Includes:
- Adapter implementation (app/src/adapter/sveltekit/)
- Test suite
- Working example (examples/sveltekit/)
- Package exports and types

examples/sveltekit/svelte.config.js

@@ -0,0 +1,12 @@
+import adapter from "@sveltejs/adapter-auto";
+import { vitePreprocess } from "@sveltejs/vite-plugin-svelte";
+
+/** @type {import('@sveltejs/kit').Config} */
+const config = {
+  preprocess: vitePreprocess(),
+  kit: {
+    adapter: adapter(),
+  },
+};
+
+export default config;