fix(cst): correct document.valueRange.end

[ikatyang]

I'd like to get the position of directives end marker (---) and contents end marker (...) but it's hard to locate them from the generated CST.

This is the minimal change I can think of, hope it's acceptable.

[eemeli]

That seems like a rather radical extension of the API, for relatively little benefit. I probably won't have time to look into this for the next two weeks or so, but CST nodes have a (so far) undocumented valueRange property that might work for this; I can't recall exactly how it's determined for CSTDocument, but it's probably the { start, end }range that would be just inside the document markers. And if it's not, it should probably be fixed to be so.