Add tests

[DylanSp]

I probably want to add tests at some point, for some mix of the following reasons:

• Unit tests for tricky game logic
• Unit/component tests for component code
• e2e tests with Cypress

Redux Toolkit suggests using Vitest and React Testing Library. Their template at https://github.com/reduxjs/redux-templates/tree/master/packages/vite-template-redux shows the necessary configuration:

• dependencies and scripts in package.json
• Vite configuration in vite.config.ts
• ESlint file patterns in .eslintrc.json to lint files such as Button.test.tsx or Game.spec.ts
• Adding vitest/globals to compilerOptions.types in tsconfig
• The src/setupTests.ts file
• Utilities in utils/test-utils.tsx