Populate attributes for the Next.js insights page for Turbopack

### Description

We require two attributes on certain spans for the Next.js insights page:

* `sentry.nextjs.function.type` (for example `"Page"`, `"Layout"`, `"generateMetadata"`)
* `sentry.nextjs.function.route` (for example `"/foo/bar/[id]/page"` or `"/foo/bar/[id]/layout`)

The spans that should have these attributes are the default export functions for Page, Layout, etc. files, and all generation functions.

This probably just involves taking spans emitted by Next.js and transforming them to have the right attributes.