Local assets example throws an error on image upload

When running the `examples/assets-local` with `pnpm dev` and try to upload an image I get:

```txt
An error occurred while resolving input fields.
- Post.banner: require() of ES Module /home/jneidel/tmp/keystone/node_modules/.pnpm/file-type@19.0.0/node_modules/file-type/index.js from /home/jneidel/tmp/keystone/packages/core/src/lib/assets/createImagesContext.ts not supported.
Instead change the require of index.js in /home/jneidel/tmp/keystone/packages/core/src/lib/assets/createImagesContext.ts to a dynamic import() which is available in all CommonJS modules.
```


`node -v => v16.20.2`

Reproduce the failure in examples/assets-local with pnpm dev and an image upload, then inspect packages/core/src/lib/assets/createImagesContext.ts where the error originates. Confirm the fix by running the example on Node v16.20.2 and verifying that image uploads complete without the module-resolution error.

Written by the indexing model from the issue text.