Add example for Strapi type extension

examples/kitchensink-ts/index.d.ts

@@ -0,0 +1,7 @@
+import '@strapi/strapi';
+
+declare module '@strapi/strapi' {
+  interface StrapiInterface {
+    customAddMethod(n1: number, n2: number): number;
+  }
+}

examples/kitchensink-ts/src/index.ts

@@ -0,0 +1,15 @@
+export default {
+  register({ strapi }: { strapi: Strapi }) {
+    // No error when trying the access & set the
+    strapi.customAddMethod = (n1, n2) => {
+      return n1 + n2;
+    };
+  },
+
+  bootstrap({ strapi }: { strapi: Strapi }) {
+    const result = strapi.customAddMethod(12, 32);
+
+    // This should print "42"
+    console.log(result);
+  },
+};