Extend addField handling in FieldTransformer to support non-field instances like Heading and Divider

In the src/Transformer/FieldTransformer.php file, the current implementation only checks for instances of Field to process through the addField method. This implementation is restrictive as there are other types of objects that need processing, such as Heading and Divider, which are not strictly fields but still require similar handling.

Issue

The code snippet:

if ($subField instanceof Field) {
    $this->addField($subField);
}

This condition only allows objects of type Field to be processed. However, instances of Heading or Divider are also valid inputs that need to be added but are currently being excluded.

Suggested Fix

We propose extending the condition to include Heading and Divider types. This can be implemented by modifying the existing conditional check to include these types, or by implementing a new interface that these classes can inherit, representing all types that are valid for addField.

A possible modification might look like this:

if ($subField instanceof Field || $subField instanceof Heading || $subField instanceof Divider) {
    $this->addField($subField);
}

Alternatively, defining a new interface like FieldCompatible could make the system more flexible and maintainable:

interface FieldCompatible {}

class Field implements FieldCompatible {}
class Heading implements FieldCompatible {}
class Divider implements FieldCompatible {}

if ($subField instanceof FieldCompatible) {
    $this->addField($subField);
}

Start in src/Transformer/FieldTransformer.php and inspect the addField call guarded by the Field instance check, then review how Heading and Divider are defined and used. Done means valid Heading and Divider instances are accepted and processed through addField without excluding existing Field inputs.