fix: .d.ts definitions for core and app (#2929)

* fix: .d.ts definitions for core and app

* fix: updated ts types to match the schema

packages/netlify-cms-app/index.d.ts

@@ -0,0 +1,7 @@
+declare module 'netlify-cms-app' {
+  import { CMS } from 'netlify-cms-core';
+
+  export const NetlifyCmsApp: CMS;
+
+  export default NetlifyCmsApp;
+}

packages/netlify-cms-app/package.json

@@ -6,6 +6,12 @@
   "repository": "https://github.com/netlify/netlify-cms/tree/master/packages/netlify-cms-app",
   "bugs": "https://github.com/netlify/netlify-cms/issues",
   "main": "dist/netlify-cms-app.js",
+  "files": [
+    "src/",
+    "dist/",
+    "index.d.ts"
+  ],
+  "types": "index.d.ts",
   "scripts": {
     "develop": "yarn build:esm --watch",
     "build": "cross-env NODE_ENV=production webpack",