Missing TypeScript types

[elektronik2k5]

Hi Brad,

I wanna make a PR to add these types:

declare module 'react-flex-columns' {
  // eslint-disable-next-line @typescript-eslint/ban-types
  type WithAsProp<P = {}> = {
    as?: React.ComponentType<P> | React.ElementType;
  };
  interface CommonProps extends WithAsProp {
    children?: React.ReactNode | React.ReactNode[];
    className?: string;
    split?: boolean;
    stack?: boolean;
  }
  export interface ColumnProps extends CommonProps {
    gutterUnit?: number;
    display?: boolean;
    size?: number;
    flex?: boolean;
    align?: 'left' | 'right' | 'center';
  }
  // https://github.com/bradwestfall/react-flex-columns/blob/master/src/lib/Column.js
  export const Column: React.FC<ColumnProps>;
  export interface ColumnsProps extends CommonProps {
    middle?: boolean;
    gutterSize?: number | null;
    gutters?: boolean | null;
    reverse?: boolean;
  }
  // https://github.com/bradwestfall/react-flex-columns/blob/master/src/lib/Columns.js
  export const Columns: React.FC<ColumnsProps>;
}

However, I also wanna make sure it actually works in a test, like I did here. But that'll take me more time than I currently have. So on the mean time we can at least have the type definitions here in the issue. :)

[bradwestfall]

Sure, can you make the PR