Explore ways to check package.json exports paths

[sodic]

Not tagging this as a bug since it's not a userland bug.
It's a missing check in our development flow that can (and on several occasions did) result in userland bugs. I don't know what's the appropriate label for such things.

Steps to reproduce:

1. Map an exports path in sdk/wasp/package.json to something that doesn't exist. For example:

   "./universal/types": "./dist/universal/somethingMadeUp.js",
   

2. Witness all our checks system fail to detect the error.

Why it matters

1. Building the SDK creates .d.ts files with unresolvable imports. I'm not sure why the build doesn't realize this - potentially because it doesn't use the exports field for resolving types.
2. Users import the wasp package, and TypeScript silently resolves these unresolved imports to anys.
3. Users have incorrect types. The anys propagate to the top. So, most of the times, the types aren't wrong, they're missing (that's why our todoApp build doesn't catch them).

Possible solutions

• Ideal: Find a way to enforce correct paths in the package.json exports field.
• Alternative: Find a way to e2e test our exposed types more robustly.