Move to webpack 5

[robbederks]

It looks like webpack 5 has a built-in way to load service workers, which would allow us to get rid of service-worker-loader as a dependency. This package is quite stale anyways, so this would be a nice improvement!

[robbederks]

@coleshaw interested in trying this?

[coleshaw]

Sounds like a good idea! Unfortunately I haven't done the webpack 4 to 5 migration yet, and don't currently have the bandwidth to take it on or dig into the implications of making the update...