feat(registry): add .npmrc config support

[evaliyev]

Summary

• Integrate npmrc-config-rs to resolve registry URLs from .npmrc files
• Support scoped package registries (@company:registry=https://private.example.com)
• Add authentication headers (Bearer tokens, Basic auth) for private registries
• Fallback to npm.jsr.io for @jsr/* packages when not explicitly configured
• Fixes feat(update): support custom npm registries #220

What I checked

• cargo test - 276 tests pass
• Manual test with private registry in .npmrc

[evaliyev]

@JamieMason Please take a look.

[JamieMason]

Thanks a lot @evaliyev, this new crate is a huge help.

It'll take some time to work through this PR but here are some thoughts so far:

• I would want to keep the logic for finding out what registry url to fetch in one place, this was Instance::get_update_url() previously but is spread over 2-3 places here.
• I think it's worth moving let npmrc = NpmrcConfig::load()?; above the let ctx = Context::create( line in main.rs, that data will likely be needed in other places in the future and it should also be easier to test LiveRegistryClient if we can pass it mock npmrcs to check different scenarios.
• I'm not sure yet if the url logic that was in Instance::get_update_url() is best off staying in Instance or if it should move to LiveRegistryClient, I've a feeling LiveRegistryClient might be better as it will have access to npmrc, but I'm not sure.

Hope this makes sense, I can take a look myself when I get time but if you can have a go at these refactors before then, that would be great. And then finally, with the crate having been created in such a short space of time, I'd just ask that you add as many E2E or integration tests as you can, ideally including some on Windows as well as Linux if you can.

Thanks again for this, a massive step forward on an issue I was struggling with.

[evaliyev]

Thank you @JamieMason for the review! Your comments really make sense and I will try to address them in the next couple of days.