Implement dependency checks inside webpack config

As a result of our recent 1.1 release issues, it might be a good idea to introduce some way of validating that we are using the correct Webpack version.

We could do this by adding some code to attempt to read a `package.json` file and looking for a `webpack` key under dependencies/devDependencies. If we see a value >= 5.0.0 ([semver](https://www.npmjs.com/package/semver)) then we abort the build.

This would then mean our CI fails and we can check what went wrong, rather than it silently "failing" (incorrectly building for our needs) and us tagging broken releases.

Start at the webpack configuration entry point and inspect how the project currently loads package.json or handles build failures. Use the dependency declarations described in the issue as the input, and verify that CI aborts for webpack versions >= 5.0.0 while supported versions continue to build.

Written by the indexing model from the issue text.