Use renovatebot to keep dependencies up to date

In the root of this repo, we seem to have a lot of out-o-date dependencies:

added 981 packages from 846 contributors and audited 4883 packages in 38.903s
found 11 vulnerabilities (4 low, 4 high, 3 critical)

We should use something like [renovate](https://renovatebot.com) to keep dependencies up to date.