DEV: adds includes helper to templates (#18259)

Usage :

```
{{#if (includes fooArray foo)}}
```

```
{{#if (includes fooString foo)}}
```

app/assets/javascripts/discourse/tests/addons/truth-helpers/integration/helpers/includes-test.js

@@ -0,0 +1,43 @@
+import { assert, module, test } from "qunit";
+import { setupRenderingTest } from "discourse/tests/helpers/component-test";
+import { render } from "@ember/test-helpers";
+import { hbs } from "ember-cli-htmlbars";
+import { exists } from "discourse/tests/helpers/qunit-helpers";
+
+module("Addons | truth-helpers | Integration | includes", function (hooks) {
+  setupRenderingTest(hooks);
+
+  test("when using an array", async function () {
+    this.foo = [1];
+    this.bar = 1;
+    await render(
+      hbs`{{#if (includes foo bar)}}<span class="test"></span>{{/if}}`
+    );
+
+    assert.ok(exists(".test"), "it returns true when element is found");
+
+    this.bar = 2;
+    await render(
+      hbs`{{#if (includes foo bar)}}<span class="test"></span>{{/if}}`
+    );
+
+    assert.notOk(exists(".test"), "it returns false when element is not found");
+  });
+
+  test("when using a string", async function () {
+    this.foo = "foo";
+    this.bar = "f";
+    await render(
+      hbs`{{#if (includes foo bar)}}<span class="test"></span>{{/if}}`
+    );
+
+    assert.ok(exists(".test"), "it returns true when element is found");
+
+    this.bar = "b";
+    await render(
+      hbs`{{#if (includes foo bar)}}<span class="test"></span>{{/if}}`
+    );
+
+    assert.notOk(exists(".test"), "it returns false when element is not found");
+  });
+});

app/assets/javascripts/truth-helpers/addon/helpers/includes.js

@@ -0,0 +1,7 @@
+import Helper from "@ember/component/helper";
+
+export function includes(params) {
+  return params[0].includes(params[1]);
+}
+
+export default Helper.helper(includes);