set up unit testing

.gitignore

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+node_modules

jest.config.js

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+export default {
+  testEnvironment: 'node',
+  transform: {},
+};
+

jsconfig.json

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+{
+  "compilerOptions": {
+    "module": "ESNext",
+    "moduleResolution": "node",
+    "target": "ESNext",
+    "types": ["jest"]
+  },
+  "exclude": ["node_modules"]
+}
+

package.json

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+{
+  "name": "lemonade",
+  "version": "1.0.0",
+  "description": "A lemonade stand game",
+  "type": "module",
+  "scripts": {
+    "test": "node --experimental-vm-modules node_modules/jest/bin/jest.js"
+  },
+  "devDependencies": {
+    "@types/jest": "^30.0.0",
+    "jest": "^30.2.0"
+  }
+}