When resolving a `TextRange` to a DOM `Range` there is an edge case
where the `start` or `end` position have an `offset` of `0` and the
element contains no text nodes.

Before this commit `toRange` would throw an `Offset exceeds text length`
error. A more useful behavior though is:

 - For the `start` position, to resolve it to the start of the next text node after
   `this.start.element`

 - For the `end` position, to resolve it to the end of the previous text
   node before `this.end.element`

This commit implements this behavior by first adding a `dir` option to
`TextPosition.resolve` to control what happens when the position's
offset is 0 and the element has no text and then specifying this option
when calling `this.{start, end}.resolve` in `TextRange.toRange`.