This removes the special comment part of the ES6 module syntax,
opting to enable ES6 module syntax by default.
It also appends `.js` to all import paths to better support in-browser
loading.
This commit moves all the input-related files from `core/`
to `core/input/`, and renames a couple as relevant
(input.js --> input/devices.js, keyboard.js --> input/util.js).