[5.x] Add depth parameter to flatten modifier

[godismyjudge95]

This PR passes through the a depth parameter to the underlying collect()->flatten($depth) call inside the flatten modifier. It still defaults to infinite if no depth is specified.

Example Data:

$input = [
    'ingredients' => [
        'spices' => ['garlic', 'cumin', 'ginger', 'turmeric', 'paprika', 'curry powder'],
        'vegetables' => ['tomatoes', 'onion'],
        'meat' => ['chicken'],
    ],
];

Example Template:

{{ data | flatten(1) | to_json }}

Example Output:

[
    ["garlic", "cumin", "ginger", "turmeric", "paprika", "curry powder"],
    ["tomatoes", "onion"],
    ["chicken"]
]