Switch type of `root` argument from `Node` to `Element`

This will be useful for switching the position selector anchoring code to use the
`TextRange` class. The one place where a non-Element node was passed was
from `anchoring/pdf.js`, a context where we can easily create a dummy
element.

src/annotator/anchoring/html.js

@@ -21,7 +21,7 @@ async function querySelector(anchor, options = {}) {
  * It encapsulates the core anchoring algorithm, using the selectors alone or
  * in combination to establish the best anchor within the document.
  *
- * @param {Node} root - The root element of the anchoring context.
+ * @param {Element} root - The root element of the anchoring context.
  * @param {Selector[]} selectors - The selectors to try.
  * @param {Object} [options]
  *   @param {number} [options.hint]
@@ -89,7 +89,7 @@ export function anchor(root, selectors, options = {}) {
 }
 
 /**
- * @param {Node} root
+ * @param {Element} root
  * @param {Range} range
  */
 export function describe(root, range) {

src/annotator/anchoring/pdf.js

@@ -330,7 +330,8 @@ function findInPages(pageIndexes, quoteSelector, positionHint) {
   const offset = getPageOffset(pageIndex);
 
   const attempt = ([content, offset]) => {
-    const root = { textContent: content };
+    const root = document.createElement('div');
+    root.textContent = content;
     const anchor = TextQuoteAnchor.fromSelector(root, quoteSelector);
     if (positionHint) {
       let hint = positionHint.start - offset;

src/annotator/anchoring/types.js

@@ -94,7 +94,7 @@ export class RangeAnchor {
  */
 export class TextPositionAnchor {
   /**
-   * @param {Node|TextContentNode} root
+   * @param {Element} root
    * @param {number} start
    * @param {number} end
    */
@@ -105,7 +105,7 @@ export class TextPositionAnchor {
   }
 
   /**
-   * @param {Node} root
+   * @param {Element} root
    * @param {Range} range
    */
   static fromRange(root, range) {
@@ -113,7 +113,7 @@ export class TextPositionAnchor {
     return TextPositionAnchor.fromSelector(root, selector);
   }
   /**
-   * @param {Node} root
+   * @param {Element} root
    * @param {TextPositionSelector} selector
    */
   static fromSelector(root, selector) {
@@ -141,7 +141,7 @@ export class TextPositionAnchor {
  */
 export class TextQuoteAnchor {
   /**
-   * @param {Node|TextContentNode} root - A root element from which to anchor.
+   * @param {Element} root - A root element from which to anchor.
    * @param {string} exact
    * @param {Object} context
    *   @param {string} [context.prefix]
@@ -153,7 +153,7 @@ export class TextQuoteAnchor {
     this.context = context;
   }
   /**
-   * @param {Node} root
+   * @param {Element} root
    * @param {Range} range
    */
   static fromRange(root, range) {
@@ -162,7 +162,7 @@ export class TextQuoteAnchor {
   }
 
   /**
-   * @param {Node|TextContentNode} root
+   * @param {Element} root
    * @param {TextQuoteSelector} selector
    */
   static fromSelector(root, selector) {