Add number.ToWords() support for large numbers in French

[louis-z]

Fixes #988

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[clairernovotny]

Can we do this without a breaking change? New overloads are fine, but renaming existing methods will break people.

[louis-z]

Can we do this without a breaking change? New overloads are fine, but renaming existing methods will break people.

Absolutely. I will adapt the PR accordingly.

[louis-z]

Actually the modified classes are internal. Do you still want me to prevent API breakage?