Use PHP CS with the PSR standards

[brunoocasali]

We should introduce stricter rules to maintain this repository and avoid "I think/I like" kind of decisions.

Maybe not all the standards we will have to be compliant https://www.php-fig.org/psr/.

I propose to adopt these standards:

• 1
• 3
• 4
• 7
• 18

Also, we should make it automated from the CI standing point.

• Add the PSR check to the CI.

[94noni]

This could be great to have indeed, before releasing v1 perhaps ( ref #265 )

[brunoocasali]

I would appreciate it if someone could provide a PR for that!

[norkunas]

• For PSR1 - I'd say to stick to Symfony CS as this is a bundle for Symfony :)
• PSR3 - currently this bundle does not log anything, but as most Symfony setups are using Monolog which is PSR3 compatible, so there are no problems I think to use it if needed.
• We already follow PSR4 with composer.
• PSR7/18 - used in meilisearch/meilisearch-php
• PSR15 is not needed