* feat: Set which browser to launch via PUPPETEER_PRODUCT
This change introduces a PUPPETEER_PRODUCT environment
variable as a first step toward using Puppeteer with
many different browsers. Setting PUPPETEER_PRODUCT=firefox, for
example, enables Firefox-specific Launcher settings.
The state is also exposed as `puppeteer.product` in the API
to support adding other product-specific behaviour as needed.
The bulk of the change is a refactoring in Launcher
to decouple generic browser start-up from product-specific
configuration.
Respecting the puppeteer-core restriction for PUPPETEER_
environment variables, lazily instantiate the Launcher
based on a `product` Puppeteer.launch option, if available.
* test: Distinguish Juggler unit tests from Firefox
The funit script is renamed to fjunit (j for Juggler, which is
used only by the experimental puppeteer-firefox package.
In contrast, the funit script now refers to running Puppeteer
unit tests against the main puppeteer package with Firefox.
To do so with Firefox Nightly, run:
`BINARY=path/to/firefox npm run funit`
A number of changes in this patch make it easier to run
Puppeteer unit tests in Mozilla's CI.
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