feat: Localize Numbers

[TurtIeSocks]

This adds support for localizing numbers, both in its own individual component I18nNumber and via interpolation for the I18nText component with a new method with_num_arg. This utilizes a couple of crates from the icu4x project, namely icu_decimal and fixed_decimal.

In this crate, they are behind the numbers feature but it is enabled by default since this is a pretty standard behavior. If you don't need it though, set default-features = false and you can save yourself some external dependencies.