Should we support relative URLs for data sources?

[LeaVerou]

I just removed the code that resolved relative URLs in parseURL() against location, since that broke Node (there's no location there) and we weren't actually using them.

Are they useful? If so, what should they resolve against?

We need an absolute URL to even pick a backend when using Backend.from(), but they may be useful when using the constructors directly.
E.g. if they resolve to the backend's base URL, we could do things like

let backend = new GithubFile("madatajs/madata/file.json")

Instead of today’s

let backend = new GithubFile("https://madatajs/madata/file.json")