Reserve room for the truncation character in preferTruncationOnSpace

[spokodev]

preferTruncationOnSpace picks the cut point assuming the truncation character occupies a single column. With a wider truncationCharacter (for example the common '...', width 3) the result exceeds columns.

import cliTruncate from 'cli-truncate';

cliTruncate('unicorns are awesome dragons here today', 6, {
	position: 'end',
	preferTruncationOnSpace: true,
	truncationCharacter: '...',
});
//=> 'unico...'  (width 8, budget 6)

cliTruncate('unicorns are awesome dragons here today', 6, {
	position: 'start',
	preferTruncationOnSpace: true,
	truncationCharacter: '...',
});
//=> '...here today'  (width 13, budget 6)

The start and end preferTruncationOnSpace branches hard-code length - columns + 1 and columns - 1, reserving a single column for the truncation character. The non-preferTruncationOnSpace paths already reserve stringWidth(truncationCharacter), and the middle branch already uses truncationWidth. This brings the two branches in line so the result stays within columns.

With the default '…' (width 1) the output is unchanged.

Adds a regression test asserting stringWidth(result) <= columns for start/end + preferTruncationOnSpace with a multi-column truncation character.