fix: replace superagent-proxy with direct use of proxy-agent

[robbkidd]

Which problem is this PR solving?

• Resolves #380

Short description of the changes

This is the equivalent behavior of the previous agent configuration. If a proxy URL is provided in libhoney config, a single ProxyAgent will be created with that proxy URL set for all connections. No nuanced lookup of proxy config from the environment based on target URL protocol.

proxy-agent is a hefty import, but we were importing an early edition of it (v5.0.0) already via superagent-proxy. Update: whew, bundlephobia really doesn't handle direct links well.

TODO?

• updates to rollup configs
  • [x] replace superagent-proxy with proxy-agent
  • [x] omit proxy-agent from browser bundle?
• [ ] maybe some more thorough proxy testing?

[kyle-watershed]

@robbkidd thanks for putting up this fix! any movement on this? I'd like to use libhoney-js with the fix as soon as this is merged and released