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/vite.config.ts

@@ -0,0 +1,6 @@
+import { sveltekit } from "@sveltejs/kit/vite";
+import { defineConfig } from "vite";
+
+export default defineConfig({
+  plugins: [sveltekit()],
+});