Remove unused dependency

`@types/loader-utils` is unused in the production output, meaning they can be safely moved
into `devDependencies`.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Closes GH-67.

package.json

@@ -38,7 +38,6 @@
   "dependencies": {
     "@rollup/pluginutils": "^4.0.0",
     "@types/estree-jsx": "^0.0.1",
-    "@types/loader-utils": "^2.0.0",
     "astring": "^1.6.0",
     "estree-util-build-jsx": "^2.0.0",
     "estree-util-is-identifier-name": "^2.0.0",
@@ -68,6 +67,7 @@
     "@mdx-js/react": "2.0.0-next.8",
     "@theme-ui/preset-base": "^0.10.0",
     "@types/babel__core": "^7.0.0",
+    "@types/loader-utils": "^2.0.0",
     "@types/node": "^16.0.0",
     "@types/react": "^17.0.0",
     "@types/react-dom": "^17.0.0",