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.
