Add phpdoc to TransformableData.php

[matthiasweiss]

I was running into an issue with phpstan on level 10, when I try to create an object using the toArray() method.

The code I have looks like this:

Model::create($data->toArray())

This results in the following error:

Model>>::create() expects array<string, mixed>, array<mixed> given.

Adding phpdoc to the definition of the class should fix this issue.
Since I was already touching this concern, I decided to add the phpdoc to the other methods returning arrays as well.

[rubenvanassche]

Technically it kan return array<array-key, mixed>, can you change that?

[matthiasweiss]

Alright, seems as if this is far more complex than I'd have hoped :(