RFC Add common mark support

[SerafimArts]

Descriptions the description requires rendering support in the markdown.

"""
This is an example of list:
- Some
- Any
"""
type Example {}

Current behaviour:

\var_dump($example->getDescription());

// string(...) "This is an example of list:\n- Some\n- Any"

Required behaviour:

\var_dump($example->getDescription());

// string(...) "This is an example of list:\n- Some\n- Any"


$renderer = new HtmlRenderer();
\var_dump($example->getDescription($renderer));

// string(...) "<p>This is an example of list:</p><ul><li>Some</li><li>Any</li></ul>"

The method signature must be updated:

public function getDescription(Renderer $renderer = null): string
{
    return $renderer ? $renderer->render(/* description */) : /* description */;
}

Related spec:

https://github.com/facebook/graphql/blob/b0b517360e5eee43f337ce7006078ed4907a83ab/spec/Section%202%20--%20Language.md#descriptions