Feature: Support supplying proxy info from `package.json`

[rschristian]

What is the current behaviour?
Setting up a proxy needs to be done in the preact.config.js

What is the motivation or use case for changing this behaviour?
CRA users can get tripped up on this

Describe the solution you'd like
Should be simple for us to support consuming proxy info in the same way. Would allow for easier migration for CRA users.