chore(doclint): support classes inheritance (#935)

This patch:
- gives meaningful names to doclint tests
- supports classes inheritance in documentation linter. When class A
  extends class B, all methods of class B are added to documentation of
  class A.

This is a prerequisite for Object Handles: ElementHandle will be
extending ObjectHandle.

References #382

utils/doclint/check_public_api/test/diff-properties/doc.md

@@ -0,0 +1,5 @@
+### class: Foo
+
+#### foo.a
+
+#### foo.c

utils/doclint/check_public_api/test/diff-properties/foo.js

@@ -0,0 +1,7 @@
+class Foo {
+  constructor() {
+    this.a = 42;
+    this.b = 'hello';
+    this.emit('done');
+  }
+}

utils/doclint/check_public_api/test/diff-properties/result.txt

@@ -0,0 +1,3 @@
+[MarkDown] Method not found: Foo.constructor()
+[MarkDown] Non-existing property found: Foo.c
+[MarkDown] Property not found: Foo.b