Implement dependency checks inside webpack config

[davwheat]

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) 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.