Closes #17780

The main issue that is fixed here: the boundary detection logic currently excludes newlines due to !at_newline:

let found = !left.is_whitespace() && !at_newline && (is_word_end || is_subword_end);

But removing && !at_newline reveals a secondary issue with the backtracking implemented in the subword motion:

let mut new_point = map.clip_point(new_point, Bias::Left);
if need_backtrack {
    *new_point.column_mut() -= 1;
}

If new_point represents the newline, an invalid position, clipping the point with a left bias causes a one-character backtrack to match a valid position. Then in the next statement we backtrack again. Because of this the cursor no longer moves once it reaches a newline.

Here the fix is to use the same approach as in the regular word motion: find an exclusive boundary instead of an inclusive one in the first place, then we don't need backtracking. The need_next_char logic, also copied from the regular motion implementation, is needed to ensure that motions that don't allow crossing line breaks still make some progress at newlines.

This whole fix is self-contained in the first commit. The second commit is a refactor that makes both motions use the same implementation (with an is_boundary predicate), since they are now so close. This way it'll be easier to keep them in sync.

Release Notes:

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