Add typescript declaration file

[Tim Green]

❯ npm run tsc
> tsc --noEmit

<source> - error TS7016: Could not find a declaration file for module 'react-countdown-clock'. '<project>/node_modules/react-countdown-clock/build/react-countdown-clock.js' implicitly has an 'any' type.
  Try `npm i --save-dev @types/react-countdown-clock` if it exists or add a new declaration (.d.ts) file containing `declare module 'react-countdown-clock';`

2 import ReactCountdownClock from 'react-countdown-clock';
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