Add typings for the package 'koa-cookie' (#41714)

* Add typings for the package koa-cookie

* format with 4 spaces

types/koa-cookie/index.d.ts

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+// Type definitions for koa-cookie 1.0
+// Project: https://github.com/varunpal/koa-cookie
+// Definitions by: John Hou <https://github.com/dancon>
+// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// TypeScript Version: 2.3
+
+/* =================== USAGE ===================
+
+    import cookie = require("koa-cookie");
+    import Koa = require('koa');
+
+    const app = new Koa();
+    app.use(cookie());
+
+ =============================================== */
+
+import { Middleware } from 'koa';
+
+declare module 'koa' {
+    interface BaseContext {
+        cookie: Record<string, any>;
+    }
+}
+
+declare function cookie(): Middleware;
+
+export = cookie;

types/koa-cookie/koa-cookie-tests.ts

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+import Koa = require('koa');
+import cookie = require('koa-cookie');
+
+const app = new Koa();
+
+app.use(cookie());
+
+app.use((ctx) => {
+    const cookie = ctx.cookie;
+    console.log(cookie);
+});
+
+app.listen(8080);

types/koa-cookie/tsconfig.json

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+{
+  "compilerOptions": {
+    "module": "commonjs",
+    "lib": ["es6"],
+    "noImplicitAny": true,
+    "noImplicitThis": true,
+    "strictNullChecks": true,
+    "strictFunctionTypes": true,
+    "baseUrl": "../",
+    "typeRoots": ["../"],
+    "types": [],
+    "noEmit": true,
+    "forceConsistentCasingInFileNames": true
+  },
+  "files": [
+		"index.d.ts",
+		"koa-cookie-tests.ts"
+	]
+}

types/koa-cookie/tslint.json

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+{
+  "extends": "dtslint/dt.json"
+}