[Demo] Fix build

[djhi]

Problem

Although the demo seems to build fine, it actually doesn't work when you run vite preview. It seems it loads multiple versions of react-router. This is only an issue in our monorepo setup.

Solution

Add aliases so that Vite correctly resolve the react-router versions we want in the demo (v7)

How To Test

• make run-demo => check it runs fine in development
• make build-demo => check it still builds fine
• cd examples/demo && yarn preview => check the production build runs fine

Additional Checks

• The PR targets master for a bugfix, or next for a feature