feat: add `isCommitted/isDiscarded/run` to `fork`

This gives more insight into the state of a fork and also gives you the ability to run another function in the context of the fork. SvelteKit needs this

.changeset/lemon-walls-judge.md

@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+feat: add `isCommitted/isDiscarded/run` to `fork`

packages/svelte/src/index.d.ts

@@ -366,6 +366,18 @@ export interface Fork {
 	 * Discard the fork
 	 */
 	discard(): void;
+	/**
+	 * Whether the fork has been committed
+	 */
+	isCommitted(): boolean;
+	/**
+	 * Whether the fork has been discarded
+	 */
+	isDiscarded(): boolean;
+	/**
+	 * Run a function within the forked context.
+	 */
+	run(fn: () => void): Promise<void>;
 }
 
 export * from './index-client.js';

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

@@ -1014,6 +1014,37 @@ export function fork(fn) {
 				batches.delete(batch);
 				batch.discard();
 			}
+		},
+		isCommitted: () => committed,
+		isDiscarded: () => !committed && !batches.has(batch),
+		run: async (fn) => {
+			if (committed) {
+				// TODO error instead?
+				await settled;
+				fn();
+			} else {
+				// We want to start with the current state of the world, not the
+				// state back when the fork was created. Also important to
+				// correctly revert state changes later
+				const previous = batch.previous;
+				batch.previous = new Map();
+				batch.activate();
+
+				flushSync(fn);
+
+				// revert state changes
+				for (var [source, value] of batch.previous) {
+					source.v = value;
+				}
+
+				// merge 'previous' map
+				// TODO is this correct?
+				for (const [source, value] of previous) {
+					if (!batch.previous.has(source)) {
+						batch.previous.set(source, value);
+					}
+				}
+			}
 		}
 	};
 }

packages/svelte/types/index.d.ts

@@ -363,6 +363,18 @@ declare module 'svelte' {
 		 * Discard the fork
 		 */
 		discard(): void;
+		/**
+		 * Whether the fork has been committed
+		 */
+		isCommitted(): boolean;
+		/**
+		 * Whether the fork has been discarded
+		 */
+		isDiscarded(): boolean;
+		/**
+		 * Run a function within the forked context.
+		 */
+		run(fn: () => void): Promise<void>;
 	}
 	/**
 	 * Returns an [`AbortSignal`](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal) that aborts when the current [derived](https://svelte.dev/docs/svelte/$derived) or [effect](https://svelte.dev/docs/svelte/$effect) re-runs or is destroyed.