Provide ICU `SpoofChecker`’s `MIXED_NUMBERS` and `HIDDEN_OVERLAY` constants

[MatTheCat]

Description

For now you can work around by using their value:

• MIXED_NUMBERS (since ICU 58): 128
• HIDDEN_OVERLAY (since ICU 62): 256

But it would be better to add these to PHP’s Spoofchecker.