Supply end position in warnings and errors

[jamesbirtles]

using the onwarn and onerror hooks you can get the start position of the error/warning, but theres currently no way to know the end position. This would be useful for editor extensions visually show where the error is.

From a quick search it looks like most if not all places that call validator.warn get the position from a node which has a start and end position, so I don't think it would be too difficult to add.

[Rich-Harris]

this was closed via #1250