[Refactor][Number Field][Slider] Extract custom logic around generic units

[Westbrook]

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Description of issue

Could the extra state (_forcedUnit) and duplication of logic (between NumberField & Slider) be factored out by following a strategy pattern wrapping NumberFormatter/NumberParser? Alternatively, we could extend those classes in @internationalized/number to support these extra units.

Either way, to the consuming code in NumberField & Slider, there would just be a single NumberFormatter/NumberParser interface, with the error catching and forced unit fallback encapsulated inside.

There may also be room to encapsulate the local caching/language resolution logic in this way as well so it is more uniformly shared across the library. A possible use case for https://lit.dev/docs/composition/controllers/