feat(typescript): Add TypeScript declaration file. (#475)

index.d.ts

@@ -0,0 +1,19 @@
+declare module 'react-element-to-jsx-string' {
+  import { ReactNode } from 'react';
+
+  interface ReactElementToJSXStringOptions {
+    displayName?: (element: ReactNode) => string;
+    filterProps?: string[];
+    showDefaultProps?: boolean;
+    showFunctions?: boolean;
+    functionValue?: (fn: any) => any;
+    tabStop?: number;
+    useBooleanShorthandSyntax?: boolean;
+    maxInlineAttributesLineLength?: number;
+    sortProps?: boolean;
+    useFragmentShortSyntax?: boolean;
+  }
+
+  const reactElementToJSXString: (element: ReactNode, options?: ReactElementToJSXStringOptions) => string;
+  export = reactElementToJSXString;
+}

package.json

@@ -5,6 +5,7 @@
   "main": "dist/cjs/index.js",
   "module": "dist/esm/index.js",
   "browser": "dist/cjs/index.js",
+  "types": "index.d.ts",
   "scripts": {
     "build": "rollup -c",
     "build:flow": "flow-copy-source -v --ignore=*.spec.js src/ dist/cjs",