[CI] Add Dependabot to our infrastructure, test, and tooling dependencies

[unlikelyzero]

Dependabot is a great way of managing dependency testing in a hands-free way.

It has lots of utility, but we'd like to restrict it to infrastructure, test, and tooling dependencies. Because we're not using a lockfile, it's possible for individual dependencies to break between builds (and the pain falls onto the individual contributor who happens to be merging at that time).

Testing Notes

Verify that a PR was automatically triggered based on a GHA change OR eslint change

[nikhilmandlik]

Verified Fixed.

[davetsay]

verified