chore: introduce //lib/api.js (#3835)

Introduce `//lib/api.js` that declares a list of publicly exposed
classes.

The `//lib/api.js` list superceedes dynamic `helper.tracePublicAPI()` calls
and is used in the following places:
- [ASYNC STACKS]: generate "async stacks" for publicy exposed API in `//index.js`
- [COVERAGE]: move coverage support from `//lib/helper` to `//test/utils`
- [DOCLINT]: get rid of 'exluded classes' hardcoded list

This will help us to re-use our coverage and doclint infrastructure
for Puppeteer-Firefox.

Drive-By: it turns out we didn't run coverage for `SecurityDetails`
class, so we lack coverage for a few methods there. These are excluded
for now, sanity tests will be added in a follow-up.

lib/Accessibility.js

@@ -13,7 +13,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-const {helper} = require('./helper');
 
 /**
  * @typedef {Object} SerializedAXNode
@@ -390,4 +389,3 @@ class AXNode {
 }
 
 module.exports = {Accessibility};
-helper.tracePublicAPI(Accessibility);