Fix NUMBERS and DATETIME Builtins

[Ygg01]

Motivation

We don't have a good NUMBERS and DATETIME builtins function. It's modelled after fluent.rs, but there it's more of a stub, until icu is adopted.

What this means

This is a hard problem, there aren't any solution that can fit 1-to-1 to those built-ins. So it will need some plumbing, relying on C# or writing it from scratch.