Fixes subword motion incorrectly jumping to next line when near end of
line. Updates boundary detection to use exclusive boundaries with
need_next_char parameter, matching regular word motion behavior.
Refactors word and subword motion to share boundary detection logic.

Closes #17780

Release Notes:

- Fixed subword motion incorrectly jumping to the next line when near
the end of a line

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Co-authored-by: dino <dinojoaocosta@gmail.com>