[POC] Support theme-based styles in SFCs

[sharkykh]

• Change cssThemes to an array
• Update Gulpfile
• Split Webpack configurations
• Add support for theme-based styles in SFCs

It slows down the build by 40-50%.
Opening this PR as a Proof-of-Concept only at the moment.

Update:

• Add babel-loader and vue-loader caching (development builds only)
• Hide stats behind a flag (yarn [dev|build] --env.stats to show)
• Switch to ExtractCssChunks
• Use Theme class instead of separate functions
• Replace oneOf with use()
• module.noParse jQuery (because it doesn't have any imports)

[sharkykh]

Something to keep in mind:
With this configuration, we can choose which theme to build.
https://webpack.js.org/configuration/configuration-types/#exporting-multiple-configurations

If you pass a name to --config-name flag, webpack will only build that specific configuration.

For our use case:
yarn [dev|build] --config-name [dark|light] [--watch]

So for example, if our Medusa is running the dark theme, we don't need to build the light theme every time we make a change. We can instead run yarn dev --config-name dark to only build the dark theme.

At this point, it's probably a good idea to add shortcuts for these,
in the form of yarn dev:dark and yarn dev:light.

Before committing you will still be required to run either the full build with yarn dev, or just build the other theme (so if you worked on dark, run yarn dev:light)