This adds an interactive Treemap-style bundle analyzer app to the Next.js codebase and writes a copy to disk when `next build --experimental-analyze` is run.
* It's a separate app in `apps/bundle-analyzer` as `@next/bundle-analyzer-ui` that uses `output: 'export'` to export a purely static Next.js app
* These static resources are copied into Next.js's published `dist` directory in the `next` package itself. Users never depend on `@next/bundle-analyzer-ui`
* When users run `next build --experimental-analyze`, these static resources are copied into the user's `.next/diagnostics/analyze` along with the data payloads needed to visualize the user's app
In an upcoming PR, we'll add `next experimental-analyze` that will create analysis files without build artifacts, as well as optionally start a static file server.
Test Plan: Ran `next build --experimental-analyze` in a test app and ran a static file server to run it.
Will Binns-Smith