Add .graphql file type export

@cedricl Maybe I am mistaken, but I think you can have whatever file name you want.

$settings['graphql_export'] = [
  'graphql_compose_server' => [
    'graphqls' => "../$site_path/graphql/schema.graphql",
    'json' => "../$site_path/graphql/schema.json",
  ],
];

However, the default value and the example in the README.md should be something that is instantly usable.

Yeah that filename is just the default filename, incase people used a dir path in the output location.

You could make it myfile.potato
Don't really need this patch.

As sweetchuck says.