feat: improve code quality with rector rules

[shakaran]

Improve code quality over src and tests folders appling rector rules with PHP 8.1 support.

Applied rules:

• ReadOnlyPropertyRector (https://wiki.php.net/rfc/readonly_properties_v2)
• TypedPropertyFromStrictConstructorRector
• NullToStrictStringFuncCallArgRector

Type of change

• Minor: New feature (non-breaking change which adds functionality)

Checklist

• I have added/updated tests for this change

[shakaran]

Hi @paulomarg, just let me know if you need more info about the changes. This is a batch of updates and improvements (first iteration). I have pending more to send when this gets merged. Thanks!

[paulomarg]

I think these changes are ok, but if we're going to add something like this, we should also include the scripts / rules so that others can also run it.

[shakaran]

I think these changes are ok, but if we're going to add something like this, we should also include the scripts / rules so that others can also run it.

ok, no prob, after you merge this, I will send more PR and one with the rector config rule. Also I can add in a CI for the future PR