Production build breaks due to invalid import; do not import from nuxt/app directly

[IlyaSemenov]

Following #240, specifically this comment: #240 (comment),

the current module needs to be transpiled, or it breaks in SSR mode. While transpiling indeed "fixes" the problem, this is a workaround with its own issues, not a proper fix.

See discussion in nuxt/nuxt#34662.

From what I gathered, the problem is apparently that the module does this:

import { useAsyncData, useRequestHeaders } from "nuxt/app"

I suspect that useAsyncData is not supposed to be used like this because it has compile-time magic (auto-generating data key) and should not bypass the Nuxt compiler.

I see the following ways we could truly address this:

1. trpc-nuxt could be converted into a "true" module (doing defineNuxtModule and injecting runtime code with addPlugin, addComposable, etc.)
2. If there are low-level counterparts to useAsyncData and useRequestHeaders, they should be used instead. nuxt/app should not be imported from third party code, it's only supposed to be imported from within a Nuxt app.
3. Simply revert Remove outdated transpilation requirement from setup docs #241, get back to using { transpile: ['trpc-nuxt'] }, but then ideally we should provide a recipe to avoid transpiling the underlying @trpc/client, or at the very least document the gotcha (being unable to use error instanceof TRPCClientError). Or, perhaps re-export the transpiled TRPCClientError?