eui [Platform] Upgrade to webpack 5

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To unblock bringing in the MDX loader, and as general house keeping, this upgrades webpack to v5 and brings a few other dependencies current.

updated webpack & related dependencies to latest
updated webpack.config.js
updated react-docgen-typescript.js with new webpack dev server env variable
updated prop-loader.js to build a fresh TS program when processing file changes, propagating those changes to props tables
removed file-loader configuration, was present to handle importing font files which we no longer do. Still present in package.json as it is a peerDep for other loaders
updated src/themes/charts/webpack.config.js
- removed the isProduction flag, was never run in that mode
- tested generation of dist/eui_charts_theme.js , confirmed JSON object of new and old matches exactly

Remaining before taking out of draft:

[x] verify custom babel things are unchanged
[x] react refresh works the same
[x] double check dev server config
- [x] contentBase (confirmed this wasn't used, has been pointing at a non-existent dir)
- [x] disabledHostCheck (confirmed access over network still work with this removed)
- [x] hot (confirmed removal, setting to true is now the default)
[x] double check loaders
- [x] cache loader
- [x] proptype loader
- [x] docgen loader
- [x] babel loader
- [x] css + sass loaders
- [x] styles-loader + css + postcss + sass loaders
- [x] file-loader
- [x] url-loader
- [x] raw-loader
[x] double check a js error in the webpack config is properly reported, now that module.exports = Promise
[x] verify src/themes/charts webpack usage works
[x] verify cypress webpack usage works (including cypress 10 branch)
[x] check in on the rebundle time

~### Checklist~