Add new pseudo-elements for customizable select

This PR adds the ::check and ::select-icon pseudo-elements so that
select elements can have a standardized and customizable dropdown icon
on the button and checkmarks next to options.

Fixes w3c#10908

Author: josepharhar

css-pseudo-4/Overview.bs

@@ -1634,6 +1634,30 @@ Styling form control pickers: the ''::picker()'' pseudo-element</h3>
 &lt;/style>
   </pre>
 
+<h3 id="select-arrow">
+Select dropdown icon: the ''::select-arrow'' pseudo-element</h3>
+
+  The ''::select-arrow'' pseudo-element only matches when the <a>originating
+  element</a> is a <{select}> element. When its computed 'content' value is not
+  ''content/none'', this pseudo-element generates a box as if it was an
+  immediate child of its <a>originating element</a>, with content as specified
+  by 'content'.
+
+  ''::select-arrow'' is a [=part-like pseudo-element=], so there is no
+  restriction on which properties apply to the ''::select-arrow'' pseudo-element.
+
+<h3 id="check">
+Option checkmark icon: the ''::check'' pseudo-element</h3>
+
+  The '::check'' pseudo-element only matches when the <a>originating
+  element</a> is a <{option}> element. When its computed 'content' value is not
+  ''content/none'', this pseudo-element generates a box as if it was an
+  immediate child of its <a>originating element</a>, with content as specified
+  by 'content'.
+
+  ''::check'' is a [=part-like pseudo-element=], so there is no restriction on
+  which properties apply to the ''::check'' pseudo-element.
+
 
 <h2 id="interactions">
 Overlapping Pseudo-element Interactions</h2>
@@ -1986,4 +2010,4 @@ Acknowledgements</h2>
     get-computed-style-crash.html
     svg-text-selection-002.html
     text-selection.html
-  </wpt>
+  </wpt>