Add NumberValidationContract

[fazedordecodigo]

Guides:

https://github.com/andrebaltieri/Flunt/blob/main/Flunt/Validations/DecimalValidationContract.cs
https://github.com/andrebaltieri/Flunt/blob/main/Flunt/Validations/DoubleValidationContract.cs
https://github.com/andrebaltieri/Flunt/blob/main/Flunt/Validations/FloatValidationContract.cs
https://github.com/andrebaltieri/Flunt/blob/main/Flunt/Validations/IntValidationContract.cs
https://github.com/andrebaltieri/Flunt/blob/main/Flunt/Validations/LongValidationContract.cs

[imDarshanGK]

Hi @fazedordecodigo,

I'm adding a NumberValidationContract. Where should I include the references to similar validation contracts: in the code or documentation?

Thanks!