Change the context to be an unsealed array

[MidnightDesign]

Changes the context key of the log_record_array type from a sealed array to an unsealed one.

Currently, $logger->records[0]['context']['foo'] results in an error in PHPStan and Pslam because the context key is defined to only ever have (at most) an exception key, but nothing else. This change makes the context array unsealed, meaning it can hold keys other than exception.

Fixes #17

This change is IMO a better solution than #21 which widens the type too far (array<array-key, mixed>).

[MidnightDesign]

On the other hand, as @derrabus mentioned, exception can actually be any value. So maybe his PR is the more correct one. I would just change the mixed[] to the more explicit array<array-key, mixed>. (It's the same type, just a bit more explicit.)

[Seldaek]

Yup closing as I think the other PR is fine, and also I'm not sure if phpstan has support for , ... yet in arrays?