[9.x] Added callable support to operatorForWhere on Collection (#41414)

* [9.x] Added callable support to operatorForWhere on Collection

Added `callable` support to `operatorForWhere`.

Currently only Collection methods `where` and `whereFirst` are used without `callable` support.

Adding callable to `operatorForWhere` allow complex usages as:

```php
$city = $cities->firstWhere(fn ($city) => $city->state->available && ($city->state->id === $stateId));
```
or
```php
$state = $states->firstWhere(fn ($state) => (bool)$state->cities->firstWhere('id', $cityId));
```

* Added callable test to `where` and `firstWhere` collection methods

* Fixed firstWhere callable test typo

Traits/EnumeratesValues.php

@@ -311,7 +311,7 @@ public function every($key, $operator = null, $value = null)
     /**
      * Get the first item by the given key value pair.
      *
-     * @param  string  $key
+     * @param  callable|string  $key
      * @param  mixed  $operator
      * @param  mixed  $value
      * @return TValue|null
@@ -548,7 +548,7 @@ public function unlessNotEmpty(callable $callback, callable $default = null)
     /**
      * Filter items by the given key value pair.
      *
-     * @param  string  $key
+     * @param  callable|string  $key
      * @param  mixed  $operator
      * @param  mixed  $value
      * @return static
@@ -995,13 +995,17 @@ protected function getArrayableItems($items)
     /**
      * Get an operator checker callback.
      *
-     * @param  string  $key
+     * @param  callable|string  $key
      * @param  string|null  $operator
      * @param  mixed  $value
      * @return \Closure
      */
     protected function operatorForWhere($key, $operator = null, $value = null)
     {
+        if (is_callable($key)) {
+            return $key;
+        }
+
         if (func_num_args() === 1) {
             $value = true;