Support Laravel 13 and improve basket operations

[Jord-JD]

What changed

• support Laravel 11, 12, and 13 while restoring the package runtime floor to its actual PHP 7.1 syntax requirement
• add explicit compatibility jobs for Laravel 5.5 through 13
• fix test migration registration so product fixtures exist before tests run
• automatically persist a new basket when its first item is added and reject unsaved basketable models clearly
• return the created or merged item from add() and respect Laravel morph-map aliases
• skip orphaned polymorphic rows while matching additions and provide a clear error when their price is requested
• add clear() for deleting every item in one operation
• replace Travis/Coveralls with GitHub Actions and refresh the documentation

Compatibility

Existing APIs remain valid. Laravel 5.5 remains supported, the package PHP floor is lowered from 8.1 to 7.1, and Composer selects the PHP floor required by each Laravel major.

Security triage

The two open Laravel advisories concern signed URL paths and the email validation rule, neither of which this basket library invokes. Keeping older Laravel majors installable is intentional; current applications resolve patched Laravel releases through Composer.

Validation

• git diff --check
• composer.json parsed with jq
• GitHub Actions covers every supported Laravel major from 5.5 through 13