Nuxt module: allow customisable `projectRoot` for Layers

We make use of Layers to create a monorepo setup for multiple sites.

The value of nuxt.options.rootDir is reporting the sub-site folder, but our .netlify folder is at the project root, so it would be useful to pass the value into the @netlify/dev config to this location in "shared" layer nuxt.config.ts i.e.,

netlify: {
  projectRoot: './'
}

I believe it's a quick change, I can open a PR if required.

https://github.com/netlify/primitives/blob/26411689aab3b6463620cf46a7f9b79e503ae0bf/packages/nuxt-module/src/module.ts#L54

const module = defineNuxtModule({
-  projectRoot: nuxt.options.rootDir,
+  projectRoot: options.projectRoot || nuxt.options.rootDir,
})