Add automated dependency updates (e.g. Renovate)

[outslept]

Follow-up to this discussion e18e/cli#83 :

Turns out the version of ast-grep used in here is a bit outdated (this explains why it pulls this much):

But the latest version is much lighter:

I couldn't find any automated dependency-update tooling (Renovate/Dependabot) configured in this repo. Adding Renovate would make it much easier to surface and review dependency bumps (including the ast-grep update), and generally reduce manual maintenance.

We wouldn't face this problem with renovate. So if you're happy with the "proposal" I can make a PR with a renovate config