Expose loop_control.loop_var as ansible_loop_var (#54240)

* Expose the loop_var as ansible_loop_var

* Add docs

* fix assert test

* Indicate version added

test/integration/targets/assert/assert_quiet.out.quiet.stdout

@@ -6,6 +6,7 @@ ok: [localhost] => (item=item_A)
 
 TASK [assert] ******************************************************************
 ok: [localhost] => (item=item_A) => {
+    "ansible_loop_var": "item",
     "changed": false,
     "item": "item_A",
     "msg": "All assertions passed"

test/integration/targets/loops/tasks/main.yml

@@ -325,3 +325,12 @@
   loop_control:
     extended: true
   when: item == 'banana'
+
+- name: Validate the loop_var name
+  assert:
+    that:
+      - ansible_loop_var == 'alvin'
+  loop:
+    - 1
+  loop_control:
+    loop_var: alvin