submit-with-selected-menu-item can use a form attribute instead

* For dropdowns that are within `form` elements, we need to be able
	to specify a `form` attribute that can act as the "form owner",
	since nested `form` elements are not spec-compliant
	* This mirrors the `form` attribute for standard inputs and buttons
		* See: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/input#form
	* If no `form` attribute is provided, the element continues
		operating from the assumption that there is a `form` inside the
		custom element

Author: tcannonfodder

src/elements/submit-with-selected-menu-item.js

@@ -20,7 +20,11 @@ class SubmitWithSelectedMenuItemElement extends HTMLElement {
   }
 
   get form() {
-    return this.querySelector(`:scope form`)
+    if(this.hasAttribute(`form`)) {
+      return document.getElementById(this.getAttribute(`form`))
+    } else {
+      return this.querySelector(`:scope form`)
+    }
   }
 }