[9.x] Standardise invokable rule translation functionality

[timacdonald]

This PR aligns the translation capabilities of invokable validation rules with the rest of the translation system. The initial PR had some opinions baked in that on reflection I don't believe should have been present and should instead be left to the developer to handle.

This PR:

• removes the strict nature of the invokable / callable translation. On reflection the initial PR introduced a quirk to the translation stuff that made it act differently to the rest of the system. Although I personally liked the strict nature of throwing when the key doesn't exist - it is a departure from what already exists and I think consistency is best. If this functionality is wanted, it should be implemented at the translator level.
• Adds the ability to specify replacements for messages. The standard :attribute, :index, and :position replacements are already in place, so this just adds custom replacement key capabilities:

// validation.php

return [
    'is' => 'The :attribute is not :value.',
];

// Rule class...

class IsRule implements InvokableRule
{
    public function __construct(private mixed $value)
    {
        //
    }

    public function __invoke($attribute, $value, $fail)
    {
        if ($this->value !== $value) {
            $fail('validation.translated-error')->translate([
                'value' => $this->value,
            ]);
        }
    }
}

// Form request rules...

return [
    'email' => new IsRule('tim@example.com'),
];

// error...

"The email is not tim@example.com."

• enables developers to specify on-the-fly locales for error messages...

$fail('validation.translated-error')->translate([
    'value' => $this->value,
], 'fr'); // Specifying french translation.

• and also enables translation with choices for pluralisation support.

With these features in place, this should cover all the require translation support for the new invokable / closure based rule stuff.