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/Worker.js

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 const EventEmitter = require('events');
-const {helper, debugError} = require('./helper');
+const {debugError} = require('./helper');
 const {ExecutionContext} = require('./ExecutionContext');
 const {JSHandle} = require('./JSHandle');
 
@@ -78,4 +78,3 @@ class Worker extends EventEmitter {
 }
 
 module.exports = {Worker};
-helper.tracePublicAPI(Worker);