Give all HTML button elements a type

The HTML button elements we use are all regular buttons that don't
submit form data to a server. According to
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/button#notes
those buttons should all have their type set to `button` explicitly, but
we only do that a handful of them. This commit fixes the issue by
consistently giving all our buttons the `button` type.

Co-authored-by: Calixte Denizet <calixte.denizet@gmail.com>

examples/learning/prevnext.html

@@ -9,8 +9,8 @@
 <h1>'Previous/Next' example</h1>
 
 <div>
-  <button id="prev">Previous</button>
-  <button id="next">Next</button>
+  <button id="prev" type="button">Previous</button>
+  <button id="next" type="button">Next</button>
   &nbsp; &nbsp;
   <span>Page: <span id="page_num"></span> / <span id="page_count"></span></span>
 </div>