Raise PHP to 8.0 + Add rector

[xepozz]

Added rector/rector to CI and composer.json. Mass creation refers to https://github.com/yiisoft/yii-dev-tool/pull/232

Fix #112

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@eb3fad2). Click here to learn what that means. Patch has no changes to coverable lines.

:exclamation: Current head 2d98f0c differs from pull request most recent head 0f9400d. Consider uploading reports for the commit 0f9400d to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #115 +/- ## ========================================= Coverage ? 99.49% Complexity ? 197 ========================================= Files ? 5 Lines ? 397 Branches ? 0 ========================================= Hits ? 395 Misses ? 2 Partials ? 0 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft)

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