feature: Control element is now optional

The control element for `CollapsableItem` is now optional. If there is no control element, you can either expand / collapse the item programmatically from JS, or you can use an external link.

src/CollapsableItem.ts

@@ -41,7 +41,7 @@ export class CollapsableItem {
 		const controlElements = element.querySelectorAll<HTMLElement>(collapsable.options.control)
 		const boxElements = element.querySelectorAll<HTMLElement>(collapsable.options.box)
 
-		if (!controlElements.length || !boxElements.length) {
+		if (!boxElements.length) {
 			throw new Error(`Collapsable: Missing control or box element.'`)
 		}