Add init command

[halvardssm]

It would be great to have an init command for CI and in general to set the @jsr:registry=https://npm.jsr.io line in the .npmrc file without having to add a package.

I would suggest providing a global option if one wants to update the users config file, otherwise adds it in the local project.

I am happy to provide a PR for this.

[marvinhagemeister]

WIP PR that I didn't manage to finish for the launch #11