[9.x] Add alpha_num_ascii validation rule

[fuwasegu]

The alpha_num rule must be used to validate that the field consists entirely of alphabetic and numeric characters.
However, in cultures such as Japan, where multibyte characters are used routinely, full-width alphanumeric characters such as Ａ (unicode: \uff21) or １ (unicode: \uff11) may be accepted as input instead of half-width alphanumeric characters such as A (unicode: A) or 1 (unicode: 1).
This can be converted to half-width alphanumeric characters programmatically, but it would be better if FormRequest could check it and return it as an error.

This PR implements a validation rule that checks for AlphaNum and single-byte alphanumeric characters (i.e., ASCII).

As an alternative, you can write regex:/^[a-zA-Z0-9]$/, but it would be easier to use if it is defined as a rule.

Usage

public function rules(): array
{
    return [
        'attribute_1' => ['alpha_num'], // 🙆‍♀ ABC123, 🙆‍♀ ＡＢＣ１２３
        'attribute_2' => ['alpha_num_ascii'], // 🙆‍♀ ABC123, 🙅‍♀ ＡＢＣ１２３
    ];
}

[taylorotwell]

You can easily write a custom closure or class based rule for this. ✅