fix: make custom element tag optional in typings (#13172)

Was missing from #12754 where we made the tag optional

fixes #13161

Author: dummdidumm

packages/svelte/elements.d.ts

@@ -1934,7 +1934,7 @@ export interface SvelteHTMLElements {
 			| string
 			| undefined
 			| {
-					tag: string;
+					tag?: string;
 					shadow?: 'open' | 'none' | undefined;
 					props?:
 						| Record<