feat: handle form attribute

[maelanleborgne]

Closes zenstruck/browser#166

Pretty rough implementation of a feature that is central for our project tests, and that we currently circumvent by abusing reflection methods. It would be very nice to have this feature shipped in the library.

I'll let GPT describe the PR :

Description

This pull request refactors the Form and Element classes to improve handling of form elements, particularly those with explicit form attributes. It updates the logic for identifying a form associated with an element.

Refactoring and Enhancements:

• src/Dom/Node/Form.php: Added the findNodesForForm method to refine how form-related nodes are identified. This method accounts for both direct descendants and nodes explicitly referencing the form via the form attribute. Updated fields, buttons, and submitButtons methods to use findNodesForForm for improved accuracy.
• src/Dom/Node/Form/Element.php: Enhanced the form method to prioritize elements with explicit form attributes before falling back to the closest form ancestor.

Test Coverage:

• tests/Node/FormTest.php: Added unit tests to validate the behavior of forms with and without IDs, ensuring correct identification of associated fields and buttons.

[maelanleborgne]

There is probably a better way to filter nodes than using reduce but my understanding of xpath is almost 0. If we can use Nodes::filter matching only on current nodes (not descendants) instead it would be great.