Update `web-app/tsconfig.json` to follow best practices for an app

[sodic]

Same as #2259 but for .wasp/out/web-app.

Slight modifications are possible due the fact that users aren't expected to work with the code in .wasp/out/server.

Check sdk/tsconfig.json (updated in #2656) for help. Also remove inheritance from @tsconfig/node20.