Add parameter types to Data ArrayAccess methods

[Strift]

The Data class implements ArrayAccess but lacks explicit parameter types in its method signatures.

File: src/Contracts/Data.php

Action:
Update the following method signatures to include mixed types:

1. offsetSet($offset, $value): void -> offsetSet(mixed $offset, mixed $value): void
2. offsetExists($offset): bool -> offsetExists(mixed $offset): bool
3. offsetUnset($offset): void -> offsetUnset(mixed $offset): void
4. offsetGet($offset): mixed -> offsetGet(mixed $offset): mixed