Add support for isInstanceOf[T] on raw JS classes.

Implemented with JavaScript 'instanceof' operator.

Also supports primitives, i.e., js.Number, js.String,
js.Boolean and js.Undefined, using typeof tests.

Based on an initial implementation by @lihaoyi.

isInstanceOf[T] is still not supported if T is a raw
JS *trait* (not class), e.g., js.Function0[_] or js.Any
itself, since these types have no existence whatsoever
in JavaScript in the first place.