Rename yml to yaml

[OndraM]

easy-coding-standard.yaml wasn't previously autodiscovered by ECS (meaning you would have to define the path always), but is now, so we can switch to the recommended extension.

[TomasVotruba]

I'd be careful about the validity of resources: https://stackoverflow.com/questions/21059124/is-it-yaml-or-yml#comment65070554_21059124

I prefer common sense with standard on Github - coverals.yml, travis.yml, yaml-sort-checker.yml...

This makes also point:
https://sourceforge.net/p/yaml/mailman/message/25795979/

While it may not seem a big deal, when one is frequently programming
for YAML files, it is much less troublesome and easier on the eyes to
write:

File.read('foo.yml')

then

File.read(Dir.glob('foo.{yml,yaml}'))

all the time.

E.g. this change caused impossible to use default value in Symfony Command, because 2 values are now "default".

[OndraM]

Important reason is that Symfony 4 switched to .yaml as default, in accordance with yaml.org recommendation (which is probably the only authoritative source regarding this).

So if you create Symfony 4 app, you will hardly anywhere use .yml (you don't configure Travis, Coveralls etc. for application). easy-coding-standard.yml would be the only place where you use .yml, which makes no sense to me...

[TomasVotruba]

That is internal naming 1 framework, not root config standard.

Also, decision by popular project based on outdated source doesn't make the source valid.

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I think is the new "tabs vs spaces" 😄 so let's stop before it becomes a thing and settle to prefer whatever we like.