Fix @ember/string dependency

`@ember/string` must be a dependency 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`.

Author: anehx

package.json

@@ -34,6 +34,7 @@
   "dependencies": {
     "@csstools/postcss-sass": "^5.0.1",
     "@ember/render-modifiers": "^2.1.0",
+    "@ember/string": "^3.1.1",
     "@ember/test-waiters": "^3.1.0",
     "@glimmer/component": "^1.1.2",
     "@glimmer/syntax": "^0.87.1",
@@ -105,7 +106,6 @@
     "@babel/plugin-proposal-decorators": "^7.23.6",
     "@babel/preset-env": "^7.23.6",
     "@ember/optional-features": "^2.0.0",
-    "@ember/string": "^3.1.1",
     "@ember/test-helpers": "^3.2.1",
     "@embroider/test-setup": "^3.0.3",
     "@fullhuman/postcss-purgecss": "^4.0.3",