This PR renames `String.endPos` to `String.rawEndPos`, as in a future
release the name `String.endPos` will be taken by the function that is
currently called `String.endValidPos`.
This PR allows Lean's parser to run with a final position prior to the
end of the string, so it can be invoked on a sub-region of the input.
This has applications in Verso proper, which parses Lean syntax in
contexts such as code blocks and docstrings, and it is a prerequisite to
parsing the contents of Lean docstrings.