Add class as an alias for className

This adds class as an alias for className to be
more consistent and intuitive. This is now possible
since 'class' is no longer a globally reserved word
in Javascript.

fixes #1310

dom.bs

@@ -6172,7 +6172,8 @@ interface Element : Node {
   readonly attribute DOMString tagName;
 
   [CEReactions] attribute DOMString id;
-  [CEReactions] attribute DOMString className;
+  [CEReactions] attribute DOMString class;
+  [CEReactions] attribute DOMString className; // legacy alias of .class
   [SameObject, PutForwards=value] readonly attribute DOMTokenList classList;
   [CEReactions, Unscopable] attribute DOMString slot;
 
@@ -6786,8 +6787,8 @@ claims as to whether using them is conforming or not.
 <p>The <dfn attribute for=Element><code>id</code></dfn> attribute must <a for=Attr>reflect</a>
 "<code>id</code>".
 
-<p>The <dfn attribute for=Element><code>className</code></dfn> attribute must
-<a for=Attr>reflect</a> "<code>class</code>".
+<p>The <dfn attribute for=Element><code>class</code></dfn> and <dfn attribute
+for=Element><code>className</code></dfn> attribute must <a for=Attr>reflect</a> "<code>class</code>".
 
 <p>The <dfn attribute for=Element><code>classList</code></dfn> getter steps are to return a
 {{DOMTokenList}} object whose associated <a for=/>element</a> is <a>this</a> and whose associated
@@ -10506,6 +10507,7 @@ Philippe Le Hégaret,
 Piers Wombwell,
 Pierre-Marie Dartus,
 prosody—Gab<!-- riel --> Vereable<!-- Gaston --> <!-- Croft -->Context(,
+Psychpsyo,
 Rafael Weinstein,
 Rakina Zata Amni,
 Richard Bradshaw,