feat: provide extends directive for configs

[antongolub]

This PR brings declarative config extension via extends references. Just like tsconfig, eslint, typedoc and many others.

• Accepts: string | Record<any, any> | Array<string | Record<any, any>>
  • 'foo' → require('foo')
  • './foo.js' → require(path.resolve(, 'foo.js'))
  • ['foo', 'bar'] → merge(base, ...extras.load(...))
  • [{"extends": "./base.json", "foo": "bar"}] → ...

closes #977

[antongolub]

@huntie, could you clarify this point?

The actual cosmiconfig preset defines searchPlaces without .yml and .es6, but it adds appropriate loaders. This looks inconsistent. What is the desired behavior?

const explorer = cosmiconfig('metro', {
  searchPlaces: [
    'metro.config.js',
    'metro.config.cjs',
    'metro.config.json',
    'package.json',
  ],
  loaders: {
    '.json': cosmiconfig.loadJson,
    '.yaml': cosmiconfig.loadYaml,
    '.yml': cosmiconfig.loadYaml,
    '.js': cosmiconfig.loadJs,
    '.cjs': cosmiconfig.loadJs,
    '.es6': cosmiconfig.loadJs,
    noExt: cosmiconfig.loadYaml,
  },
});