Allowed placing of ambiguous-width characters

Characters with the East Asian Ambiguous Unicode property are allowed to be placed on the canvas. In some fonts, they may be rendered to span over multiple cells.

Steps to reproduce:

1. artscii put a0 0 1 Ⅷ (Character used for demonstration is U+2167 Roman Numeral Eight. For a list of ambiguous-width characters, look here.)
2. View canvas with e.g. Source Code Pro font.

Expected behavior:

Ambiguous-width character is not put on the canvas and an error message is produced.

Actual behavior:

Ambiguous-width character is put on the canvas.