feat: Implement JavaScript Coverage (#1673)

This patch introduces a new `page.coverage` namespace with two methods:
- `page.coverage.startJSCoverage` to initiate JavaScript coverage
  recording
- `page.coverage.stopJSCoverage` to stop JavaScript coverage and get
  results

test/assets/jscoverage/involved.html

@@ -0,0 +1,15 @@
+<script>
+function foo() {
+  if (1 > 2)
+    console.log(1);
+  if (1 < 2)
+    console.log(2);
+  let x = 1 > 2 ? 'foo' : 'bar';
+  let y = 1 < 2 ? 'foo' : 'bar';
+  let z = () => {};
+  let q = () => {};
+  q();
+}
+
+foo();
+</script>

test/assets/jscoverage/multiple.html

@@ -0,0 +1,2 @@
+<script src='script1.js'></script>
+<script src='script2.js'></script>

test/assets/jscoverage/ranges.html

@@ -0,0 +1,2 @@
+<script>
+function unused(){}console.log('used!');</script>

test/assets/jscoverage/script1.js

@@ -0,0 +1 @@
+console.log(3);

test/assets/jscoverage/script2.js

@@ -0,0 +1 @@
+console.log(3);

test/assets/jscoverage/simple.html

@@ -0,0 +1,2 @@
+<script>
+function foo() {function bar() { } console.log(1); } foo(); </script>

test/assets/jscoverage/sourceurl.html

@@ -0,0 +1,4 @@
+<script>
+console.log(1);
+//# sourceURL=nicename.js
+</script>

test/assets/jscoverage/unused.html

@@ -0,0 +1 @@
+<script>function foo() { }</script>