Add TypeScript definition file about react-flag-icon-css

[jonfreedman]

Here's a starting point

declare module 'react-flag-icon-css' {
    import {PureComponent} from 'react';

    export interface Options {
        useCssModules: boolean;
    }

    export interface FlagIconProps {
        code: string;
        size?: 'lg' | '2x' | '3x' | '4x' | '5x';
        flip?: 'horizontal' | 'vertical';
        rotate?: 30 | 60 | 90 | 180 | 270;
        squared?: boolean;
        className?: string;
        styleName?: string;
    }

    export class FlagIcon extends PureComponent<FlagIconProps> {
    }

    export default function FlagIconFactory(react: any, opts?: Options): () => FlagIcon
}

Although as per the various issues around string literal types (e.g. microsoft/TypeScript#20271) you end up needing to write something like this

<FlagIcon code='eu' size={'lg' as 'lg'}/>

[jonfreedman]

Types are now @ https://www.npmjs.com/package/@types/react-flag-icon-css

Please feel free to merge them back into the lib if appropriate

[matteocng]

Hi @jonfreedman thanks for that! It is unfortunate that TypeScript has some issues around string literal types (Flow too), I don't know if we could make it typecheck computed props too, that would be pretty nice (size={someSize}). If you could make a PR please feel free to do so, personally I was looking into typescript-definition-tester to run a test suite, but there may be better or more standard ways. Then, after the tests worked (even without custom flags initially), the enum types could be generated like we do with generate:types for Flow.

[matteocng]

Thanks again @jonfreedman, that is great! 👍