Typescript: add type definitions

Author: calling

index.d.ts

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+export { default as loadScript } from './loadScript';
+export { default as loadScriptPromised } from './loadScriptPromised';

loadScript.d.ts

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+// Needs a name due to a bug in the ESLint TS parser
+export default function defaultExport(src: string, cb: Function, attrs?: object): void;

loadScriptPromised.d.ts

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+// Needs a name due to a bug in the ESLint TS parser
+export default function defaultExport(url: string): Promise<void>;