Upgrade existing Blitz apps with newapp template changes

### What do you want and why?

As a developer, I would like to be able to seamlessly upgrade my blitz project version using the cli, so that I can easily gain new features, bug fixes, and so on.

### Possible implementation(s)

1. modify docs and existing `blitz new` flow on file conflict
2. create an altogether new cli command `blitz upgrade` or similar

### Additional context

1. current upgrade pattern seems to be like `npx blitz@latest new myApp` where you intentionally overwrite your existing app and resolve conflicts.
2. Problematically, the current upgrade pattern seems to autocommit
3. Another problem is that it seems the developer must manually and correctly remember their `new myApp` input selections (js vs ts, picking which form, etc)