improve the rnx-kit CLI generator to have a scope for webapis new packages

We already have a way of generating new packages for the rnx-kit monorepo (https://github.com/microsoft/rnx-kit/blob/main/scripts/new-package.mjs) but we could use adding an extra option to the CLI ex. `--scope` to indicate when we want to create a generic package for rnx-kit VS when it would need to be a webapi one. There shouldn't be that many differences, but it'd be a nice improvement.