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/golden/csscoverage-involved.txt

@@ -0,0 +1,16 @@
+[
+  {
+    "url": "http://localhost:8907/csscoverage/involved.html",
+    "ranges": [
+      {
+        "start": 149,
+        "end": 297
+      },
+      {
+        "start": 327,
+        "end": 433
+      }
+    ],
+    "text": "\n@charset \"utf-8\";\n@namespace svg url(http://www.w3.org/2000/svg);\n@font-face {\n  font-family: \"Example Font\";\n  src: url(\"./Dosis-Regular.ttf\");\n}\n\n#fluffy {\n  border: 1px solid black;\n  z-index: 1;\n  /* -webkit-disabled-property: rgb(1, 2, 3) */\n  -lol-cats: \"dogs\" /* non-existing property */\n}\n\n@media (min-width: 1px) {\n  span {\n    -webkit-border-radius: 10px;\n    font-family: \"Example Font\";\n    animation: 1s identifier;\n  }\n}\n"
+  }
+]