readline: move charLengthLeft() and charLengthAt()

This moves the charLengthLeft() and charLengthAt() into the internal
readline file. This allows sharing the functions internally with
other code.

PR-URL: https://github.com/nodejs/node/pull/31112


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>