get index(): T_LIST {
    return this._get_axis(0);
  }
Return the index values of the `DataFrame`

@returns {List}

@example
const df = new DataFrame([{x: 1, y: 2}, {x: 2, y: 3}, {x: 3, y: 4}]);

// Returns List [0, 1, 2, 3]
df.index;