fix: Remove ember-cli-string-helpers

- This pulls @ember/string 3.1.1 into dependent project and that causes a lot of issues
- Since we have `capitalize` in @ember/string, we can just call that in our internal helper
- Also bumped internal version of @ember/string to v4.x

Fixes ember-learn#1666

Author: MichalBryxi

addon/helpers/capitalize.js

@@ -0,0 +1,6 @@
+import { helper } from '@ember/component/helper';
+import { capitalize } from '@ember/string';
+
+export default helper(function capitalizeHelper(positional) {
+  return capitalize(positional[0]);
+});

app/helpers/capitalize.js

@@ -0,0 +1 @@
+export { default } from 'ember-cli-addon-docs/helpers/capitalize';

package.json

@@ -53,7 +53,6 @@
     "ember-cli-clipboard": "^1.2.1",
     "ember-cli-htmlbars": "^6.3.0",
     "ember-cli-postcss": "^8.2.0",
-    "ember-cli-string-helpers": "^6.1.0",
     "ember-cli-string-utils": "^1.1.0",
     "ember-cli-version-checker": "^5.1.2",
     "ember-code-snippet": "^3.0.0",
@@ -111,7 +110,7 @@
     "@ember-data/serializer": "^5.3.9",
     "@ember-data/store": "^5.3.9",
     "@ember/optional-features": "^2.2.0",
-    "@ember/string": "^3.1.1",
+    "@ember/string": "^4.0.0",
     "@ember/test-helpers": "^3.3.1",
     "@embroider/test-setup": "^3.0.3",
     "@fullhuman/postcss-purgecss": "^4.1.3",