feat(launcher): add option to run Puppeteer with different browsers (#5137)

* 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.

experimental/puppeteer-firefox/.cirrus.yml

@@ -7,7 +7,7 @@ task:
     dockerfile: .ci/node8/Dockerfile.linux
   xvfb_start_background_script: Xvfb :99 -ac -screen 0 1024x768x24
   install_script: npm install
-  test_script: npm run funit
+  test_script: npm run fjunit
 
 task:
   name: node8 (macOS)
@@ -19,7 +19,7 @@ task:
     - brew install node@8
     - brew link --force node@8
   install_script: npm install
-  test_script: npm run funit
+  test_script: npm run fjunit
 
 # task:
 #   allow_failures: true
@@ -28,4 +28,4 @@ task:
 #    os_version: 2016
 #  name: node8 (windows)
 #  install_script: npm install --unsafe-perm
-#  test_script: npm run funit
+#  test_script: npm run fjunit