[5.x] Add keys and values modifiers

[godismyjudge95]

This PR adds two modifiers keys and values to complement the already robust array of array modifiers (ie. flip, flatten, sort, etc.)

Example Data:

$data = [
    'key1' => 'Value 1',
    'key2' => 'Value 2',
];

Example Template:

{{ data | keys | to_json }}

{{ data | values | to_json }}

Example Output:

['key1', 'key2']

['Value 1', 'Value 2']