feat(tests): add basic arithmetic functions and corresponding tests

- Implemented `add` and `subtract` functions in multiple packages (web, ui, utils).
- Created unit tests for these functions using Vitest in respective test files.
- Updated turbo.json to include a new test pipeline for coverage and watch mode.

apps/docs-dev/package.json

@@ -24,7 +24,7 @@
         "@storybook/react": "^8.2.6",
         "@storybook/react-vite": "^8.2.6",
         "@vitejs/plugin-react": "^5.1.2",
-        "eslint": "^8.57.1",
+        "eslint": "^8.57.0",
         "serve": "^14.2.1",
         "storybook": "^8.2.6",
         "typescript": "5.5.4",