* chore: extract `BrowserRunner` into its own module
`src/Launcher.ts` is large and hard to work in. It has multiple objects
defined in it:
* ChromeLauncher
* FirefoxLauncher
* BrowserRunner
* Launcher
This change moves BrowserRunner into its own module. More refactorings
like this will follow but this is the first step.
Node.js v6 was end-of-life'd in April, 2019, with AWS Lambda prohibiting updaets to the Node.js v6 runtime since June 30, 2019.
This makes it quite safe for us to remove the Node 6 support from the repository.
SourceFactory was meant to cache Sources so that they could be used
in different preprocessor tasks.
This turned out to be over-engineering. This patch kills the layer.
