Move core-js to dev dependency
Move core-js to dev dependency

This was added in https://github.com/webpack/webpack/pull/11308, but is only
required for tests. It doesn't need to be installed as a regular dependency for
end users.

SimenB/webpack,webpack/webpack,SimenB/webpack,SimenB/webpack,SimenB/webpack,webpack/webpack,webpack/webpack,webpack/webpack