feat(pivot): add support for multiple pivot types when using the same accessor

[pataar]

Summary

Currently, when you have multiple belongsToMany relations with a custom pivot, the accessor will have a single type. This PR ensures that multiple types are allowed. Also the $pivot property should be nullable as it is not always set during runtime.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Misc. change (internal, infrastructure, maintenance, etc.)

Checklist

• Existing tests have been adapted and/or new tests have been added
• Add a CHANGELOG.md entry
• Update the README.md
• Code style has been fixed via composer fix-style

[rpatew2c]

There's a small issue with the docblocks this PR generates. If the type is already present, it is duplicated. Here is a sample from my codebase running the latest 3.2.1 of ide-helper

- * @property-read \Modules\Contacts\Infrastructure\Models\Phonable|\Modules\Contacts\Infrastructure\Models\Emailable|\Modules\Contacts\Infrastructure\Models\Mailable|null $pivot
+ * @property-read \Modules\Contacts\Infrastructure\Models\Phonable|\Modules\Contacts\Infrastructure\Models\Phonable|\Modules\Contacts\Infrastructure\Models\Emailable|\Modules\Contacts\Infrastructure\Models\Emailable|\Modules\Contacts\Infrastructure\Models\Mailable|\Modules\Contacts\Infrastructure\Models\Mailable|null $pivot