Implement some lessons learned from webextension-toolbox

[adambullmer]

I like the way this repo solves some of the single pipeline for cross-browser challenges. For example:

• Create an output per browser
• Use an ENV var to specify which browser is the target for a single build, and make a convenience build job to iterate over each vendor.
• Exposing which browser is being built in process.env so that developers can do specific things per target, and let tree-shaking and dead code deletion optimizations take over
• Allowing for tagging manifest keys as for specific browser (vendor) targets: __chrome|opera__key,
  • I think I would prefer inverting the targets so that it becomes more readable which it actually is: description__chrome|opera__
  • Could use the WebextensionPlugin if I don't invert the vendor and key syntax.
• Conditional inclusion of the browser polyfill to further reduce bundle size in browsers that don't need it