Add @contentHtmlTag (#1038)

Author: mkszepp

ember-basic-dropdown/src/components/basic-dropdown-content.gts

@@ -48,7 +48,7 @@ export interface BasicDropdownContentSignature {
     dir?: string;
     defaultClass?: string;
     overlay?: boolean;
-    htmlTag?: keyof HTMLElementTagNameMap;
+    htmlTag?: keyof HTMLElementTagNameMap | undefined;
     onFocusIn?: (dropdown?: Dropdown, event?: FocusEvent) => void;
     onFocusOut?: (dropdown?: Dropdown, event?: FocusEvent) => void;
     onMouseEnter?: (dropdown?: Dropdown, event?: MouseEvent) => void;

ember-basic-dropdown/src/components/basic-dropdown.gts

@@ -66,6 +66,7 @@ export interface BasicDropdownArgs {
   preventScroll?: boolean;
   matchTriggerWidth?: boolean;
   triggerHtmlTag?: keyof HTMLElementTagNameMap;
+  contentHtmlTag?: keyof HTMLElementTagNameMap;
   onInit?: (dropdown: Dropdown) => void;
   registerAPI?: (dropdown: Dropdown | null) => void;
   onOpen?: (dropdown: Dropdown, e?: Event) => boolean | void;
@@ -544,6 +545,7 @@ export default class BasicDropdown extends Component<BasicDropdownSignature> {
           preventScroll=@preventScroll
           rootEventType=(or @rootEventType "click")
           vPosition=this.vPosition
+          htmlTag=@contentHtmlTag
           destination=this.destination
           destinationElement=this.destinationElement
           top=this.top