strip LTM and RTM entities when normalize text

To support bidirectional text alphabetization (https://en.wikipedia.org/wiki/Bidirectional_text)

Via EbookFoundation/free-programming-books#6714

Author: davorpa

Diff for: lib/alphabetical-list-items.js

@@ -3,8 +3,8 @@ const visit = require('unist-util-visit');
 const toString = require('mdast-util-to-string');
 
 function normalize(text) {
-  const removeAtBeginning = /^([-._(《"'])*/;
-  const removeInside = /[,:]/;
+  const removeAtBeginning = /^([-._(《"'\u200e\u200f])*/;
+  const removeInside = /[,:\u200e\u200f]/;
   const replaceWithSpace = /-/;
   return text.toLowerCase()
     .trim()