UAX44: loose matching of symbolic values and the `is` prefix

[Mathias Bynens]

> 
>> The `is` prefix doesn’t provide any functionality that would otherwise
>> be unavailable. It doesn’t add any value, yet causes incompatibility,
>> author confusion, and it increases implementation complexity.
> 
> I don't see any evidence that it adds no value. Support for existing
> implementations is value.

It adds no value because it doesn’t enable any new functionality.
I agree support for existing implementations would have some value, but given that existing implementations disagree on the properties for which they support `is` that is not going to happen anyway. It’s impossible to be compatible with all those different implementations at the same time.