chore: add `batch.id` property (#17831)

I cannot tell you how many times I have temporarily added this code to
make it easier to debug some async stuff. I am extremely bored of doing
so. I'm just going to add it to `main` to save myself the annoyance. We
can remove it once everything async is stable

packages/svelte/src/internal/client/reactivity/batch.js

@@ -76,7 +76,12 @@ export let is_flushing_sync = false;
  */
 export let collected_effects = null;
 
+let uid = 1;
+
 export class Batch {
+	// for debugging. TODO remove once async is stable
+	id = uid++;
+
 	/**
 	 * The current values of any sources that are updated in this batch
 	 * They keys of this map are identical to `this.#previous`