Implement non-zero dimension attribute parsing

Fixes #8445

The only attributes I found that we have implemented that uses non-zero
dimenion attributes:

* `width` for `<td>` and `<th>` (table cells)
* `width` for `<table>`

I updated these implementations to use the new non-zero dimension
attribute parsing and added associated regression tests.

tests/wpt/web-platform-tests/html/rendering/non-replaced-elements/tables/table-width.html

@@ -0,0 +1,30 @@
+<link rel="match" href="table-width-ref.html">
+
+<style>
+table {
+  border-collapse: collapse;
+}
+
+td {
+  padding: 0;
+}
+</style>
+
+<!-- width=0 should be treated as 'auto' -->
+<table width=0>
+  <tr>
+    <td>
+      a b
+    </td>
+  </tr>
+</table>
+
+<hr>
+
+<table width=1>
+  <tr>
+    <td>
+      a b
+    </td>
+  </tr>
+</table>