Crash if naming a Sanity schema type "imageAsset"/"fileAsset"

[rexxars]

If you make the following schema type:

{
  "name": "imageAsset",
  "type": "image",
  "fields": [{
    "name": "alt",
    "type": "string"
  }]
}

...it will be prefixed with Sanity when rewriting it to Gatsby schema, which collides with our default SanityImageAsset (sanity.imageAsset) type. I imagine the same goes for any other sanity-internal types.