[9.x] feature: min_digits and max_digits validation

[danharrin]

As per the size rule, the number of digits in numeric input cannot be validated traditionally using min and max.

Currently, we have, digits to validate the exact length of a number, and digits_between to validate the length within two boundaries.

However, as far as I can see there is not yet a way to validate that a number has a minimum OR maximum number of digits, just both.

This PR introduces two validation rules - min_digits and max_digits for this use:

Validator::validate([
    'number' => 1000,
], [
    'number' => [
        'min_digits:3', 'max_digits:5', // Passes as `1000` has 4 digits
        'min:3', 'max:5', // Fails as `1000` is greater than 5
    ],
])

[danharrin]

The failing 8.1 test is unrelated and needs rerunning.

[danharrin]

Hey @taylorotwell do you need laravel/laravel and laravel/docs PRs for this or are you handling that?

Doesn't look like anything has been pushed or PRed, so:

laravel/laravel#5975 laravel/docs#8148