set position to last line when passing + arg without number

set position to last line when passing `+` arg without number by m4rch3n1ng · Pull Request #14571 · helix-editor/helix

revive of #13865, as the author does not have the time anymore (see #13865 (comment)), with the outstanding review resolved. this still just sets the line_number to usize::MAX, but i think that sho......