This PR removes the deprecated `puppeteer.devices` in favor of a new
exported object `KnownDevices`. `devices` can also be exported, but has
been deprecated.
This PR moves the puppeteer source code into separate mono-repo packages:
- `puppeteer` and `puppeteer-core` are now separated into their own
packages.
- `puppeteer-core` has a new exports called `puppeteer-core/internal`
for internal usage.
Tests and various tools have been updated to accommodate the migration.
