Add support for regular expression validations

This patch was authored by @K1NXZ and released by @0xTim.

Validate a regular expression pattern

Example:

struct TestContent: Codable, Validatable {
    static func validations(_ validations: inout Validations) {
        validations.add("numbersOnly", as: String.self, is: .pattern("^[0-9]*$"))
    }
    
    let numbersOnly: String
    
    init(numbersOnly: String) {
        self.numbersOnly = numbersOnly
    }
}