chore: Add demo tsconfig.json, prettifies .json on commit, add preact as a devDep

[rschristian]

• Adds tsconfig.json to demo/
  • As we don't set (and can't, due to using tsc as the compilation method) the JSX config, TS will (correctly) trip up if you open the demo in an editor.
• Adds preact as a devDep
  • Was just testing Vite 5 and had to use --legacy-peer-deps as I didn't want to fiddle with versions, but unfortunately Preact was only implicitly included because it's a peer dep. Should probably be a full dev dep.
• Prettifies .json on commit else my badly formatted changes to the package.json slip through :p