fix(shadcn): use system proxy settings (#3869)

[pomeh]

Hello,

Here is a small PR in order to use system wide proxy settings for fetching the remote repository.

I used proxy-agent package, which is a "super package" on top of https-proxy-agent (among others).

The main benefit from this change is to use the correct proxy in all situations:

• if you have defined HTTPS_PROXY (uppercase) instead of https_proxy (lowercase)
• use http_proxy setting (no HTTPS), in case you've used REGISTRY_URL with an HTTP URL
• use no_proxy setting whenever it's relevant¹
• add support for dynamic proxy settings (PAC file format)

Note: I've seen the same kind of code in packages/cli source command, but I've not updated it because of its deprecation notice.

Note: I'm not yet fluent with pnpm. There a quite a few changes inside its lock file, but the only 2 commands I ran was pnpm install proxy-agent and pnpm remove https-proxy-agent.

pnpm test are running successfully (on gitpod.io).

Notes:

• ¹: this was my main problem, since https_proxy is set on my env. for all outgoing requests, but no_proxy is configured with an exception for shadcn URL, and this exception was not use within the shadcn cli

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