Allow extending config

[pi0]

// foo.config.js
export default {
 // Extend relative file
 extends: './foo.config.base', // cwd is same
 extends: '../base/foo.config', // cwd is ../base

 // Discover and extend config from directory
 extends: '../base', // loads base/foo.config.js 

 // Extend multiple config
 extends: ['../theme', '../base'],

 // Next steps
 extends: 'github:foo/bar' // clones https://github.com/foo/bar.git and loads foo.config.js
}

Resolved layers should be flattened and preserved in the final resolved configuration for custom merging/extend strategy:

const config = {
  baseColor: 'red',
  userOption: true,
  _extends: [
    {
      config: { baseColor: 'red' },
      meta: {
        cwd: 'path/to/theme',
        configFile: 'path/to/theme/foo.config.ts'
      }
    },
    {
      config: { baseColor: 'blue' },
      meta: {
        cwd: 'path/to/base',
        configFile: 'path/to/base/foo.config.ts'
      }
    }
  ]
}