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This changeset adds PHPStan on max level to the test environment for all supported PHP versions. Unlike #64 runs the maximum level supported with all errors addressed without having to resort to a baseline. The changeset size seems reasonable and this does not other affect our public API, so this should be safe to apply.

If this PR is merged, I will backport the changes from v4 to v3. The changes are almost identical across all versions, but only v4 takes advantage of fibers (#15).

Builds on top of clue/framework-x#200, clue/framework-x#201 and #66
See also https://github.com/orgs/reactphp/discussions/469 and #40