No validation on xsi:nil values

[fbricon]

Given the following snippet (I'm not sure it's actually valid or makes any sense at all):

<?xml version="1.0" ?>
<foo xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> 
    <bar xsi:nil="foo">Fred</bar> 
</foo>

xsi:nil valid values are true/false, yet there's no validation error here

[fbricon]

Now that I think of it, we should add an actual schema for xsi, under the hood, so that'd provide completion, hover and validation support for xsi without having to hard-code anything