General code cleanup (#28)

* Add .idea folder to .gitignore

* Remove useless concatenation

* Add phpunit cache directory to .gitignore

* Use iterable type instead of unnecessary checks

* Remove unnecessary null coalescing

If the return value is `null`, it will return `null` anyway. If the method doesn't exist, it will throw an `Error` because of the undefined method.

* Add typehint

* Merge meta properties in one go instead of in a loop

* Simplify control flow

* Use non-deprecated phpstan configuration value

* Add missing types

* Fix style with php-cs-fixer

* Add tests that satisfy the ArrayIterator branch

* Use is() for comparison in in()

* from(int|string), tryFrom(int|string)

---------

Co-authored-by: Samuel Štancl <samuel@archte.ch>

tests/TestCase.php

@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace ArchTech\Enums\Tests;
 
 use Orchestra\Testbench\TestCase as TestbenchTestCase;