dynamic stan extension for Model::find

[shmax]

Adding a PHPStan extension so that the various return types of Model::find can be statically checked.

In other words, if you're a PHPStan user, the following snippets will now all pass analysis without errors:

/**
 * Static checking for single model
 */
$book = Book::find(1);
assert($book instanceof Book);

$book = Book::find(['name'=>'Ubik']);
assert($book instanceof Book);

/**
 * Static checking for nullable single model
 */
$book = Book::find('first', ['name'=>'Waldo']);
assert(is_null($book));

$book = Book::find('last', ['name'=>'Waldo']);
assert(is_null($book));

/**
 * Static checking for array of models
 */
$books = Book::find(
    'all',
    ['name' => 'William']
);
assert(0==count($books));

$books = Book::find(1, 3, 8);
assert(0==count($books));

$books = Book::find([1, 3, 8]);
assert(0==count($books));

$books = Book::find(['conditions'=> ['name' => 'Kurt']]);
assert(0==count($books));