[8.x] Added prohibits validation

[samlev]

NB: I originally submitted this as prohibited_with, which was turned down because with would behave differently from required_with. I feel that changing the wording to prohibits resolves this issue without compromising on the functionality of this validation.

I've encountered a requirement in a few systems where two potential fields could be passed, but if one exists the other cannot exist. The prohibited_if validation allows you to counter this only if you know the potential values of the second field, so I've added prohibits as a way to create a simple NAND between fields.

The most recent example was working with the Facebook Business SDK, where a customer record could accept email or emails, but passing both fields causes an error (because it can't be resolved into a correct single value). With this validation, I could catch that earlier in the application and ensure that I'm not having to deal with API/SDK errors. For this example:

Validator::validate([
    'email' => 'example@example.com', 
    'emails' => ['example@example.com', 'other.example@example.com']
], [
    'email' => 'prohibits:emails'
]);

You can also pass multiple fields for a situation where you have one field or multiple fields (e.g. passing a model ID as well as fields that can only be set on creation). In this case, if any of the listed prohibits fields exist, it will trigger a validation failure.

Validator::validate([
    'post_id' => 1, 
    'created_by' => 2,
    'created_at' => '2020-07-16 20:54:22'
], [
    'post_id' => 'prohibits:created_by,created_at'
]);

Finally, this can be used with required_without to create a XOR, where one property must be passed, but never both.

Validator::validate([
    'email' => 'example@example.com', 
    'emails' => ['example@example.com', 'other.example@example.com']
], [
    'email' => ['prohibits:emails', 'required_without:emails']
]);

If you choose to merge, I will send PRs to laravel/docs and laravel/laravel.