[8.x] Add 'multiple_of' validation rule

[timacdonald]

This validation rule allows devs to validate that a given number is a "multiple of" another number. The naming for this rule comes from the corresponding JSON schema validation rule.

This validation rule can be useful if you use a number input with a step attribute.

Input...

<input type="number" step="0.5" name="foo">

Validation...

public function rules(): array
{
    return [
        'foo' => [
            'multiple_of:0.5',
        ]
    ];
}

Expected result

0 ✅
0.5 ✅
1 ✅
1.5 ✅
1.6 ❌

Would love you to just scan over the data provider and make sure it validates as you would expect.

If ya keen to merge this one I can PR the laravel/laravel repo with a translation, and the laravel/docs repo with some documentation, unless someone jumps in to does it first (anyone reading thing, feel free 👍)

[ejunker]

@timacdonald should add multiple_of to the validation docs

[tontonsb]

Why is false considered to be the correct answer for the test cases involving 10.5 and .3? 10.5 is a multiple of .3. Do you know of any browser that supports these attributes but does not allow entering 10.5 in this input?

<input type=number min=0 step=.3 >

[timacdonald]

See: #34960 (comment)