Add ncu-env-check

There's a pretty decent number of requirements to contribute to node that are easy to fumble. It would be lovely if node-core-utils provided a suite of checks that can be run with a single command (ncu-env-check) to validate that a given environment is properly configured.

Here's the list I have so far for Linux:

• system has git
• system has python3
• system has g++
• system has make
• origin is set to something that's not git@github.com:nodejs/node.git (and potentially check if it's a fork of nodejs/node for even better DX, which could be checked via the API (thanks @zkatfor the tip <3))
• upstream is set to git@github.com:nodejs/node.git
• check that git config user.name is set
• check that git config user.email is set
• check that an SSH key matching one published to GitHub is set

I'm sure there's a number of others that could be added, too.

This would be particularly useful if we are to follow through on https://github.com/nodejs/node/issues/39672, particularly for getting people up and running with minimal friction.

Start by locating the existing CLI entry points in node-core-utils and determine how a new ncu-env-check command is registered. Use the Linux checklist in this issue as the initial acceptance criteria, including Git remotes, Git identity, required tools, and the GitHub SSH key. Done means one command reports whether the environment is ready for Node.js contribution.