feat: Add support for YAML files

[christopher-kleine]

This addresses #341

I tested this with an existing config. It's also compatible with existing configs. But since I didn't invest too much into this, let me know if I forgot anything.

Another change: At start it shows the used config file (if any).

TODO:

• [x] Support for .air.yml and .air.yaml (Default: .air.toml)
• [x] air init toml, air init yaml and air init yml

[christopher-kleine]

I take my PR back. I guess I'll manage my own fork now. Maybe this way some changes get actually done.