v0.6.6

This is a compatibility release that contains backported features from the 0.7.x branch.
Once v0.7 is released, it will be the way forward for this project.

• Feature: Improve PHP 8.4+ support by avoiding implicitly nullable types.
  (#114 by @clue)

• Improve test suite to run tests on latest PHP versions and report failed assertions.
  (#113 by @clue)