[webpack config] Drop workbox support

[EvanBacon]

Why

This PR is part of a series of changes that will shift SWs to require local setup.

We don't enable service workers by default (anymore) so it's a bit unfortunate that every project needs to install the code required for adding them (which accounts for a large portion of expo cli). Workbox is also a bit slow to upgrade webpack versions, and that transitively effects our ability to upgrade. Our offline feature only supports SPA styled websites (Expo web default), which means Next.js and Gatsby users end up having to install a bunch of code that they won't be using either.

Workbox uses a lot of deprecated dependencies, which ends up adding lots of warnings to expo-cli installation. Removing workbox will drop the following install warnings when users run npm i -g expo-cli:

npm WARN deprecated @hapi/joi@16.1.8: Switch to 'npm install joi'
npm WARN deprecated @hapi/topo@3.1.6: This version has been deprecated and is no longer supported or maintained
npm WARN deprecated @hapi/hoek@8.5.1: This version has been deprecated and is no longer supported or maintained
npm WARN deprecated @hapi/pinpoint@1.0.2: Moved to 'npm install @sideway/pinpoint'
npm WARN deprecated @hapi/formula@1.2.0: Moved to 'npm install @sideway/formula'
npm WARN deprecated @hapi/address@2.1.4: Moved to 'npm install @sideway/address'

With yarn add expo-cli:

warning expo-cli > xdl > @expo/webpack-config > workbox-webpack-plugin > workbox-build > @hapi/joi@16.1.8: Switch to 'npm install joi'
warning expo-cli > xdl > @expo/webpack-config > workbox-webpack-plugin > workbox-build > @hapi/joi > @hapi/hoek@8.5.1: This version has been deprecated and is no longer supported or maintained
warning expo-cli > xdl > @expo/webpack-config > workbox-webpack-plugin > workbox-build > @hapi/joi > @hapi/topo@3.1.6: This version has been deprecated and is no longer supported or maintained
warning expo-cli > xdl > @expo/webpack-config > workbox-webpack-plugin > workbox-build > @hapi/joi > @hapi/topo > @hapi/hoek@8.5.1: This version has been deprecated and is no longer supported or maintained
warning expo-cli > xdl > @expo/webpack-config > workbox-webpack-plugin > workbox-build > @hapi/joi > @hapi/address@2.1.4: Moved to 'npm install @sideway/address'
warning expo-cli > xdl > @expo/webpack-config > workbox-webpack-plugin > workbox-build > @hapi/joi > @hapi/formula@1.2.0: Moved to 'npm install @sideway/formula'
warning expo-cli > xdl > @expo/webpack-config > workbox-webpack-plugin > workbox-build > @hapi/joi > @hapi/pinpoint@1.0.2: Moved to 'npm install @sideway/pinpoint'

This is 28% decrease in warnings.

• related PWA fails to compile due to GenerateSW plugin configuration keys being renamed #3718
• example/template feat: added workbox example examples#283

[brentvatne]