Support custom paths for blitz

### What do you want and why?

To be able to use **multiple** _alternative_ paths for blitz, it is currently locked to `app`

This allows for nicely decoupled projects. It is currently semi-possible with a bit of manual work, but having it so we can separate blitz into multiple packages means we can develop frontend pages separately, and deploy them separetly.

### Possible implementation(s)

I imagine we'll need to add a configuration into multiple parts of blitz.

- blitz.config.js: the directories to include, e.g: `"paths": [ "packages/core", "packages/example" ]`
- getBlitzModulePaths - need to include in the globbing process here

However, someone with more knowledge about the codebase might be able to pinpoint the exact locations to get started with! (and if I can get this information, I am happy to get going with it, document it, and write tests for it!)

### Additional context

n/a - please let me know how I can help with this one!

Start by reading blitz.config.js and getBlitzModulePaths, focusing on how the current app path is configured and used during globbing. Define the configuration for multiple alternative paths, update the path discovery behavior, and add tests showing that separate package paths are included.

Written by the indexing model from the issue text.