[w3c/selection-api] Selection API collapse and setBaseAndExtent algorithms contain redundant checks of offset (#124)

In the [specification of the collapse method](https://w3c.github.io/selection-api/#dom-selection-collapse) there is a step that says that the method must throw `IndexSizeError` if `offset` is longer than the node’s `length`. This check is also done as part of creating _newRange_ in subsequent steps. So the `IndexSizeError` exception will be thrown in either case. However, because the specification explicitly calls for doing this _before_ checking the node’s root, this difference is observable in obscure cases.

I suggest we remove the explicit `IndexSizeError` step. Better for consistency with other operations in the Selection API that don't explicitly specify an `IndexSizeError` check. I think it’s likely this is not already needed for web compatibility, so it’s not too late to fix this.

These same considerations apply to the [setBaseAndExtent method](https://w3c.github.io/selection-api/#dom-selection-setbaseandextent). And I suggest the same change, removing the explicit `IndexSizeError` step.

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