Add a Range object and finish the Selection binding

Hello,
This is a WIP, i'm currently testing the methods myself to be sure that everything is OK.
This follows the documentation here: [Range](https://developer.mozilla.org/en-US/docs/Web/API/range) and [Selection](https://developer.mozilla.org/en-US/docs/Web/API/Selection.extend).
Since i'm quite new to js_of_ocaml I post this PR now to gather some feedbacks and informations on what i'm supposed to do or not. :-)