_createConfigFromPackageJSON() {
    try {
      const filePath = path.resolve('./package.json');
      const packageJSON = fs.readFileSync(filePath, 'utf8').toString();
      const packageObj = JSON.parse(packageJSON);
      return packageObj.esdoc;
    } catch (e) {
      // ignore
    }

    return null;
  }
create config object from package.json.
@return {ESDocConfig|null} config object.
@private