Update @wordpress/scripts and webpack to latest versions

[ice9js]

It seems the @wordpress/scripts version provides configuration for Webpack 5 while we were still on Webpack 4 making it impossible to build anything.

This patch updates Webpack to version 5 and @wordpress/scripts to the latest version.

Testing

• Remove any existing dependencies and install the repo from scratch to prevent there being any obscure stale dependencies.
• Verify that all commands from package.json still work as expected and bundles are being built correctly.