Add JsonFileStore feature

[Cyberschorsch]

I found your great project during #hacktoberfest and gave it a spin. I noticed that there is currently only an option for saving results as CSV and thought having the option for saving to a JSON file would be a nice addition to your project.

Usage is fairly simple, instead of adding the CSV Store to your crawler just add the new store instead:

$crawler->setStore(new JsonFileStore(YOUR_STORE_PATH));

[otsch]

Hey, thanks a lot for you contribution! Good idea!
I just have a few requests:

• As i can see in the it saves Results to a JSON file test case, the resulting structure in the created file is [{...}] with one result, [[{...}, {...}]] with two results, and then [[{...}, {...}], [{...}]] with three results. Can you please change it, so it's always just an array with a list of objects in it? So [{...}] after one result, [{...}, {...}] after two, [{...}, {...}, {...}] after three, and so on.
• When I check out the branch and run the tests, they sometimes fail...not always but sometimes. I think it's probably related to running tests in parallel with pest and the beforeAll() and afterAll() methods in the two test files SimpleCsvFileStoreTest.php and JsonFileStoreTest.php. I think they probably try to create or delete folders/files at the same time. Maybe we should commit an empty _files directory with an empty .gitkeep file in it instead of always creating that directory. And also suppress errors when removing files in the afterAll() hook.
• There's one phpstan error...please check it by running composer stan and fix it.

[otsch]

Never mind. I changed it. Thanks for your contribution @Cyberschorsch! 🙏🏻🏅