Fix various Karma + webpack warnings and minor issues

[colinrotherham]

This PR includes a few follow-up fixes to Karma + webpack

Fix Karma webpack output to prevent overwriting dist/

Previously npm run karma used webpack to overwrite dist/ but using karma-webpack defaults instead. This has now been fixed so that Karma output is written to os.tmpdir() as the author intended

Fix Karma webpack karma-sourcemap-loader warnings

Karma webpack devtool option now outputs inline source maps to fix warnings about missing source maps

Ensure webpack uses new serve CLI command

We can now use the webpack CLI rather than the webpack-dev-server package binary:

npx webpack
npx webpack serve

Ensure webpack always exits on build errors

The webpack NoEmitOnErrorsPlugin plugin is now enabled by default but we turn it off in development mode to help diagnose problems in the browser rather than exit

Warnings since webpack v5 update

Additionally, new warnings have been fixed following the webpack v5 update:

• Upgrade to webpack v5 #614

Fix Karma webpack performance warnings

The Karma webpack stats: 'errors-only' option has been updated for webpack v5 to ignore performance warning recommendations since these are only useful for production builds

Fix Karma webpack DefinePlugin warning for NODE_ENV

The Karma webpack build runs with NODE_ENV=test but this logs a warning in webpack v5 because “test” does not match webpack config.mode allowed values "none" | "development" | "production"

[colinrotherham]

I did investigate webpack clean: true but it deletes the custom React and Preact bundles

It appears that webpack is unaware we override the output path to dist dist/lib so they get "cleaned"

[romaricpascal]

That's a good couple of tidy ups 🙌🏻 Tried dev, test and build commands locally and it all looked fine. ⛵