feat: Introduce config option for standardized null in typehints

[kayw-geek]

This PR introduces a new configuration option force_nullable_for_default_null to nullable_type_declaration_for_default_null_value rule, aimed at enhancing the consistency and clarity of typehints across our codebase.

By automatically converting optional types to explicit union types with null, we enable a more standardized approach to handling nullable type declarations.

Key Benefits:

• Consistency: Ensures a uniform standard across the codebase for representing nullable types, making the code more predictable and easier to read.
• Clarity: Expanding ?Type to Type|null makes the nullable aspect of the typehints explicit, improving readability, and comprehension for developers.
• Flexibility: With this configurable option, developers can tailor the behavior to their specific project needs, choosing between implicit optional types or explicit nullability.

Implementation Details:
The configuration option force_nullable_for_default_null can be toggled to automatically modify the typehints during the PHPCSFixer fix process. When enabled, it ensures that all parameters with a default null value explicitly include null in their type declaration, transforming ?Type hints to Type|null for clearer and more explicit code.

[Wirone]

Standardisation of nullable type declaration is handled with... nullable_type_declaration fixer 😉. There's no need for supporting this in nullable_type_declaration_for_default_null_value, which has different responsibility.

To achieve what this PR aims for you just need to configure nullable_type_declaration_for_default_null_value with use_nullable_type_declaration=true (default) and if you prefer union type you have to also add nullable_type_declaration with syntax=union.

Unfortunately it looks like we need to close this without merging.