fix: ensure createEventDispatcher works with types from generics

fixes #8860

This contains a small but unfortunately unavoidable breaking change: If you used `never` to type that the second parameter shouldn't be set (which the docs recommended for a short time), then you need to change that to `null`

.changeset/long-humans-dress.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure createEventDispatcher works with types from generics

documentation/docs/05-misc/03-typescript.md

@@ -96,7 +96,7 @@ Events can be typed with `createEventDispatcher`:
 	import { createEventDispatcher } from 'svelte';
 
 	const dispatch = createEventDispatcher<{
-		event: never; // does not accept a payload
+		event: null; // does not accept a payload
 		type: string; // has a required string payload
 		click: string | null; // has an optional string payload
 	}>();

documentation/docs/05-misc/04-v4-migration-guide.md

@@ -36,7 +36,7 @@ import { createEventDispatcher } from 'svelte';
 const dispatch = createEventDispatcher<{
 	optional: number | null;
 	required: string;
-	noArgument: never;
+	noArgument: null;
 }>();
 
 // Svelte version 3:

packages/svelte/src/runtime/internal/public.d.ts

@@ -80,14 +80,12 @@ export interface DispatchOptions {
 export interface EventDispatcher<EventMap extends Record<string, any>> {
 	// Implementation notes:
 	// - undefined extends X instead of X extends undefined makes this work better with both strict and nonstrict mode
-	// - [X] extends [never] is needed, X extends never would reduce the whole resulting type to never and not to one of the condition outcomes
+	// - | null | undefined is added for convenience, as they are equivalent for the custom event constructor (both result in a null detail)
 	<Type extends keyof EventMap>(
-		...args: [EventMap[Type]] extends [never]
-			? [type: Type, parameter?: null | undefined, options?: DispatchOptions]
-			: null extends EventMap[Type]
-			? [type: Type, parameter?: EventMap[Type], options?: DispatchOptions]
+		...args: null extends EventMap[Type]
+			? [type: Type, parameter?: EventMap[Type] | null | undefined, options?: DispatchOptions]
 			: undefined extends EventMap[Type]
-			? [type: Type, parameter?: EventMap[Type], options?: DispatchOptions]
+			? [type: Type, parameter?: EventMap[Type] | null | undefined, options?: DispatchOptions]
 			: [type: Type, parameter: EventMap[Type], options?: DispatchOptions]
 	): boolean;
 }

packages/svelte/test/types/actions.ts

@@ -1,4 +1,4 @@
-import type { Action, ActionReturn } from '$runtime/action';
+import type { Action, ActionReturn } from '$runtime/action/public';
 
 // ---------------- Action
 

packages/svelte/test/types/create-event-dispatcher.ts

@@ -1,7 +1,7 @@
 import { createEventDispatcher } from '$runtime/internal/lifecycle';
 
 const dispatch = createEventDispatcher<{
-	loaded: never;
+	loaded: null;
 	change: string;
 	valid: boolean;
 	optional: number | null;