Refactor Node range as [start, value-end, node-end]

[eemeli]

Fixes #258 by adding a third value to the range tuple.

For block values, value-end and node-end are always the same as the previous end value. For flow collections and documents, node-end is the same as the previous end position, while value-end marks the end of the actual value.

@gorkem, would this work for your use case?

[eemeli]

Merging, as otherwise work on error ranges gets really clumsy.