What kind of change does this PR introduce?

Fix in schema

Did you add tests for your changes?

No

If relevant, link to documentation update:

Summary

Webpack output.library.root accepts array of strings, however the schema only accepts string. Many projects depend on nesting externals under a namespace. This will eliminate the need to create custom plugins so that we can nest modules under the root name. FYI @ibezkrovnyi

For example below, --> Webpack will output root['my-global-root']['my-app-name'].

output: {
  library: {
    /* 
     schema throws an error as type is only string but
     webpack itself can accept array of strings as well
   */
    root: ['my-global-root', 'my-app-name'], 
    amd: 'my-app-name',
    commonjs: 'my-app-name'
  },
  libraryTarget: 'umd'
}

Does this PR introduce a breaking change?

No

Other information