fetch() wrapper with API similar to lua-https

[tpimh]

lua-https is going to be a part of LÖVE 12 release, and can be used on Windows, macOS and Linux platforms. It would be nice if love.js also provided a similar interface, so the code using https.request wouldn't need to be ported to be used in browser.

I believe, it's unnecessary to compile the native part of the library with Emscripten, instead only a simple wrapper for Fetch API needs to be implemented.