Allow .yaml for configuration file

[echavaillaz]

As TYPO3 prefer .yaml for YAML files it would be better to use this extension name for configuration files (or better both .yaml && .yml).

@see: https://docs.typo3.org/m/typo3/reference-coreapi/main/en-us/CodingGuidelines/CglYaml.html

Thanks

[martin-helmich]

Full ack. 👍 Simply changing yml to yaml would definitely break many setups, so -- as already noted -- it'd be necessary to support both file extensions.

I cannot make any promises as to when I'll get around to this. In the meantime, PRs are always welcome. 🙂