fix: add missing plaintext-only value to contenteditable type (#9242)

fixes #9181

Author: vhladko

.changeset/slimy-comics-switch.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: add missing plaintext-only value to contenteditable type

packages/svelte/elements.d.ts

@@ -486,7 +486,7 @@ export interface HTMLAttributes<T extends EventTarget> extends AriaAttributes, D
 	accesskey?: string | undefined | null;
 	autofocus?: boolean | undefined | null;
 	class?: string | undefined | null;
-	contenteditable?: Booleanish | 'inherit' | undefined | null;
+	contenteditable?: Booleanish | 'inherit' | 'plaintext-only' | undefined | null;
 	contextmenu?: string | undefined | null;
 	dir?: string | undefined | null;
 	draggable?: Booleanish | undefined | null;