Add hasSlot utility

src/utilities/slot.ts

@@ -35,3 +35,10 @@ export function getTextContent(slot: HTMLSlotElement): string {
 
   return text;
 }
+
+//
+// Determines whether a slot with the given name exists in an element.
+//
+export function hasSlot(el: HTMLElement, name: string) {
+  return [...el.querySelectorAll('[slot]')].filter((slottedEl: HTMLSlotElement) => slottedEl.slot === name).length > 0;
+}