When running `rails stimulus_reflex:install` on a new Rails app using
Yarn & esbuild, the following error is raised:

```
  --- [yarn] ----
  rails aborted!
  NameError: undefined local variable or method `package_json'
  for #<Rails::Generators::AppGenerator...
```

Looks like it's a missing update from
#655

## Why should this be added
Setup raises errors at the Yarn step