Make `process.env` Explicit

Changed

• Move to simple validator functions like string, boolean, number and integer
  • envvar had some issues with Error inheritance that caused stack issues
• Require sources to be passed into the package explicitly
  • Allows non-node.js environments such as webpack and Cloudflare Workers (which may just pull from variables)
• New error object
  • Shrinking overall dependency size by removing combine-errors and other error-related dependencies

Historical change log: History.md