The `Source` class is a new entry point for sources that are not plain
array or iterable. It allows accessing other features like camel-case
keys or custom paths mapping in a convenient way.
It should be used as follows:
```php
$source = \CuyZ\Valinor\Mapper\Source\Source::json($jsonString)
->camelCaseKeys()
->map([
'towns' => 'cities',
'towns.*.label' => 'name',
]);
$result = (new \CuyZ\Valinor\MapperBuilder())
->mapper()
->map(SomeClass::class, $source);
```
