[FEATURE] Add Rector

[Starkmann]

This part of #851.

In later parts, we'll add a Rector wrapper to runTests.sh and a CI job, and apply possible changes suggested by Rector.

Part of #851
Closes #125

[oliverklee]

We also should document somewhere (e.g., in the manual) how to run Rector via DDEV and runTests.sh.

Also, we should add the rector.php to the list of not-packaged files in .gitattributes and the prepare-release Composer script.

[lukaszuznanski]

@Starkmann are you willing to work on this? If so, I will close my #992

[lukaszuznanski]

Hi,

We've implemented a policy to close pull requests that have been inactive for over six months. If you're still interested in contributing and would like to continue working on this pull request, we're more than happy to keep it open.

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[coveralls]

Pull Request Test Coverage Report for Build 8987381816

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 8987315319:	0.0%
Covered Lines:	46
Relevant Lines:	46

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💛 - Coveralls

[sbuerk]

Not sure if moving rector installation to a subfolder composer.json would be feasonable directly, as for TYPO3 v13 and rector there is a conflict regard the nikic php parser librady version (for the overall working forward to TYPO3 v13).

[oliverklee]

Not sure if moving rector installation to a subfolder composer.json would be feasonable directly,

I'd prefer to do that in a separate step.