[BREAKING/v3] Upgrade to webpack 5

[LukeSheard]

Taken from the original PR #1421

Upgrade Webpack 4 -> Webpack 5.

• Configuration similarities with CRA have been maintained where it makes sense.
• Output file names are slightly changed (see tests) because now, for webpack 5, chunkFileName is only used for code splitting) (i.e. chunkFileName doesn't control vendor chunks; they're controlled by filename instead.
• Removed optimize-css-assets-webpack-plugin and introduced css-minimizer-webpack-plugin as advised in optimize-css-assets-webpack-plugin's repo:

  ⚠️ For webpack v5 or above please use css-minimizer-webpack-plugin instead.

[changeset-bot]

🦋 Changeset detected

Latest commit: 7e79670

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
modular-scripts	Major

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[coveralls]

Coverage increased (+0.3%) to 26.498% when pulling 7e79670 on feature/webpack-5 into d7c8725 on main.