Fix @ember/string dependency (#1582)

`@ember/string` must be a peer dpendency as `ember-cli-addon-docs` imports
it in their codebase. Currently this still works, but with v4 of
`@ember/string` (v2 addon) this will break as it fully relies on
`ember-auto-import`.

package.json

@@ -155,6 +155,7 @@
     "webpack": "^5.89.0"
   },
   "peerDependencies": {
+    "@ember/string": "^3.1.1 || ^4.0.0",
     "ember-data": ">= 3.0.0",
     "ember-fetch": "^8.1.1",
     "ember-source": ">= 4.0.0"