fix: prevent deprecation warning in php ^8.1

[xyng]

Description

Using this library in PHP ^8.1 causes this deprecation warning:

deprecated: 8192 :: Return type of Sanity\Transaction::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

This PR adds the mentioned annotation #[\ReturnTypeWillChange], which silences the deprecation without breaking compatibility to older PHP-Versions by specifying the return type.
The annotation is seen as a comment by older PHP-Versions.

What to review

jsonSerialized was annotated with #[\ReturnTypeWillChange] in these files:
lib/Path.php
lib/Selection.php
lib/Transaction.php

Testing

No deprecation warning present after applying the patch.