Add ::check pseudo element

This pseudo element is added to allow <option> elements inside
<select> elements to have a standardized and customizable
checkmark icon.

Spec definition:
```
Option checkmark icon: the '::check' pseudo-element

The '::check' pseudo-element is only generated when the originating
element is a option element which has an ancestor select element
with base appearance.

'::check' is a tree-abiding pseudo-element. It accepts all
properties.
It inherits from its originating element.

'::check' generates a box as if it was an child of its originating
element, preceding any boxes generated by the '::before'
pseudo-element, with content as specified by 'content'.
```

Defined in:
w3c/csswg-drafts#10986

Bug: 369319576
Change-Id: Ie5058d66f90644f9732bc619a3f587b6203d1cf9

html/semantics/forms/the-select-element/customizable-select/option-computed-style.tentative.html

@@ -11,7 +11,7 @@
 <script>
 test(() => {
   assert_equals(
-    getComputedStyle(document.querySelector('option'), '::before').content,
+    getComputedStyle(document.querySelector('option'), '::check').content,
     `"\u2713" / ""`);
 }, 'appearance:base-select options should have a checkmark with empty alt text.');
 </script>