feat(config): disallow redefining dependencies already defined in the root

## Description

I'd really like the ability to prevent having a dependency in a monorepo package that's already defined at the root.

Say I have `jest` defined in `./package.json`, syncpack could be configured to disallow `./apps/my-app/package.json` having that dependency too.

## Suggested Solution

Honestly I have no idea how you'd want to configure this. I'm happy with something as simple as the above, but I'm sure others would love to customise it further for their own needs.

## Help Needed

The ability to produce a sweet ass config interface.

Start by reviewing syncpack’s existing configuration interface and dependency validation behavior. Define how root and package dependencies should be detected, how users configure the restriction, and what tests should demonstrate that duplicate dependencies are rejected while supported customization works.

Written by the indexing model from the issue text.