C2LC-192: Configure ESLint to ignore dragdroptouch

Configure ESLint to ignore errors in the dragdroptouch library.

We do this by turning on create-react-app's support for ESLint
customization. This support is currently marked as "experimental" in
the documentation (https://create-react-app.dev/docs/setting-up-your-editor/#experimental-extending-the-eslint-config).

There is currently a caching issue in create-react-app that causes
previous ESLint configurations to interfere with future runs. A
workaround is to delete the 'node_modules/.cache/eslint-loader'
directory. See:

- facebook/create-react-app#8683
- https://webpack.js.org/loaders/eslint-loader/

.env

@@ -0,0 +1 @@
+EXTEND_ESLINT=true

package.json

@@ -27,7 +27,8 @@
         "eject": "react-scripts eject"
     },
     "eslintConfig": {
-        "extends": "react-app"
+        "extends": "react-app",
+        "ignorePatterns": ["src/vendor/"]
     },
     "browserslist": {
         "production": [

src/App.js

@@ -19,7 +19,7 @@ import * as Utils from './Utils';
 import type { DeviceConnectionStatus, Program, RobotDriver } from './types';
 import messages from './messages.json';
 import './App.scss';
-import './dragdroptouch/DragDropTouch.js';
+import './vendor/dragdroptouch/DragDropTouch.js';
 
 // Uncomment to use the FakeRobotDriver (see driver construction below also)
 //import FakeRobotDriver from './FakeRobotDriver';