feat: implement CSS Coverage (#1714)

This patch adds two new methods to the `page.coverage` namespace:
- `page.coverage.startCSSCoverage()` - to initiate css coverage
- `page.coverage.stopCSSCoverage()` - to stop css coverage

The coverage format is consistent with the JavaScript coverage.

test/assets/csscoverage/unused.html

@@ -0,0 +1,7 @@
+<style>
+@media screen {
+  a { color: green; }
+}
+/*# sourceURL=unused.css */
+</style>
+