These getters are introduced as a more convenient substitute for
a `require('puppeteer/Errors')` and
`require('puppeteer/DeviceDescriptors')`.
This way we can make cross-browser story nicer - a single require
of `puppeteer` or `puppeteer-firefox` fully defines Puppeteer
environment.
This patch refactors doclint so that more checks and more generators
could be added.
This patch:
- Introduces 'Source' class, which holds file content in-memory and
allows it to be updated.
- Introduces 'Message' class - which is a pair of a text and a type.
Messages could have either 'error' type or 'warning' type.
