Webpack should throw on non-allowlisted warnings

[grahamlangford]

Motivation

• See https://pixiebrix.slack.com/archives/C07FSHNJLTV/p1724167755751999

Acceptance Criteria

• [ ] Missing imports throws
• [ ] Webpack warnings are treated as errors
• [ ] Allow-listed warnings are ignored

Notes

• https://www.npmjs.com/package/warnings-to-errors-webpack-plugin
• https://stackoverflow.com/questions/60092487/with-webpack-is-it-possible-to-force-warnings-to-be-treated-as-errors