add strict: true compiler option

tsconfig.json

@@ -1,6 +1,7 @@
 {
   "extends": "@tsconfig/ember/tsconfig.json",
   "compilerOptions": {
+    "strict": true,
     // The combination of `baseUrl` with `paths` allows Ember's classic package
     // layout, which is not resolvable with the Node resolution algorithm, to
     // work with TypeScript.