Allow type declarations from extend layers

Adding type augmentations from the main layer is possible using top-level index.d.ts (and what we recommend in docs. We could also support it for all layers for augmenting NuxtApp (plugin injections) and AppConfigInput (app.config from themes).

[pi0]

Would you please help on supporting index.d.ts from layers? 🙏🏼

[danielroe]

I think it’s a nice idea (and possibly worth implementing), but I think there are some solutions to consider first:

1. Use other files. Any declarations you add in any file within the layer that is used in the nuxt build should already work for this purpose, and in fact I would personally recommend co-locating the definition with the usage.
2. Prefer inference. For example, plugins or app config can work this way.

For app config, the following might do it (though I haven’t yet tested):

export default defineAppConfig({
  myTheme: {} as Partial<MyThemeConfig>
})