Nombres

Convert ints into french.

Some references:

• https://leconjugueur.lefigaro.fr/frlesnombres.php
• https://www.miakinen.net/vrac/nombres
• https://www.dcode.fr/ecriture-nombre-lettres

Provided as a 1-function library, and a basic command-line utility (nbr — will certainly be renamed to something longer and less likely to name clash).