Support getting bun config

[TeddyHuang-00]

What is the problem this feature would solve?

Some package manager related program will want to respect the registry specified by users and a common way for them to do so is to call something like npm config get registry. While bun reserved bun config subcommand for future use, it hasn't been implemented in recent updates. Similar issues regarding setting the config through command line also exists: #6795, yet in my opinion allow reading can be potentially a lot easier to achieve first and may pave the way to a final solution.

What is the feature you are proposing to solve the problem?

Implement the bun config get related features first to allow reading bun configs programmatically.

What alternatives have you considered?

I do have considered parsing the configs according to bun's documentation. However after a second thought, I think this falls beyond the scope of that project and is better handled by bun for sake of consistency.