Upgrade webpack to v5

[pfftdammitchris]

I upgraded these dependencies in order to support webpack 5:

• [x] webpack 4 --> 5
• [x] webpack-cli 3 --> 4
• [x] copy-webpack-plugin 4 --> 8

I applied updates from breaking changes from migrating webpack 4 to webpack 5 in this pull request:

• [x] compilation.chunks changed from Chunk[] to Set<Chunk>
• [x] chunk.files changed from string[] to Set<string>
• [x] webpack.config.js syntax in module.rules to change plural loaders to loader, and use for multiple loaders
• [x] Updates to some typescript typings

I also changed the middleware.raw.ts raw source file to bind itself to window if it exists (because I was getting errors that browser stayed null:

[tmhrbr]

Can we get one merged please?

[dvdvdmt]

@rubenspgcavalcante please accept this PR 🙏

[rushilsrivastava]

Hi all, please check out https://github.com/SimplifyJobs/webpack-ext-reloader/. We are maintaining the webpack 5 version + rolling out manifest v3 soon! PRs welcome :)