Support Webpack 4

[nicholas-codecov]

In our initial release we're currently only supporting Webpack 5, however there is quite a large Webpack 4 community out there that we should also support.

Note

This would require supporting at a minimum Node 16, which we currently do not.

[aj-codecov]

Just adding a little context to this one, roughly 20% of webpack downloads in the last 7 days were webpack 4, so it'd be a meaningful expansion of potential users.