[Feature] Pre-1.0: Use the add-on variables functionality to seed the .env file

[jherr]

Which project does this relate to?

Create Tanstack App

Describe the feature

Some add-ons like Convex or Clerk need an API key. Use the existing variables functionality to prompt for that.

Additional context

No response

[timoconnellaus]

Things to consider:

• what env file are they using - .env, .env.local, .dev.vars, wrangler.jsonc (non secret)
• would need to also add them to t3-env
• vite vs nextjs prefixes - so it is generic for all frameworks

Maybe we should have the concept of events for this?
e.g. add-env variable event - an add-on can watch for it and managing adding it in itself

[jherr]

Maybe the add-on info contains a list of the environment variables, their types, a description, etc. And we sum those up before app creation and put those into the template system. So that add-ons like t3-env can then reflect on those are set them up properly.

Otherwise t3-env is going to become a super add-on that knows about the existence of all the other add-ons and it would just get nasty.