Roman numeral system could use the dedicated unicode characters

[NamorNiradnug]

There are "Roman Numeral ..." characters in the "Number Forms" block.

A proper implementation of this feature should produce, for example, a single character "Ⅺ" instead of two characters "ⅩⅠ" for eleven. This means that the plain NumeralSystem::Additive is not suitable.

[MDLC01]

I am not sure why using those characters would be preferable. After all, they are only encoded in Unicode as compatibility characters.¹²

Footnotes

1. https://en.wikipedia.org/wiki/Number_Forms ↩

2. https://en.wikipedia.org/wiki/Unicode_compatibility_characters ↩

[T0mstone]

Also there are single codepoints for up to Ⅻ, which is really weird imo. Would 13 be ⅫⅠ or ⅩⅢ or ⅪⅡ or ⅩⅠⅠⅠ, or any of the several other options?
In combination with what @MDLC01 said, I'd say we stay away from them and keep using regular letters.