test(compiler): add test case for nested unset with numeric string keys

- Create test for nested unset behavior with numeric string keys in empty array
- Verify proper removal of numeric string keys when using unset operation
- Confirm expected array structure after unset operations
- Document issue #114 related to numeric string key handling

tests/compiler/basic/unset-numeric-string-key.phpt

@@ -0,0 +1,34 @@
+--TEST--
+Nested unset removes numeric string keys created in an empty array (#114)
+--FILE--
+<?php
+function main(): void {
+    $a = [];
+    $a['1']['1'] = '1';
+    $a['1']['2'] = '2';
+    $a['1']['3'] = '3';
+    var_dump($a);
+    unset($a['1']['1']);
+    unset($a['1']['2']);
+    var_dump($a);
+}
+?>
+--EXPECT--
+array(1) {
+  [1]=>
+  array(3) {
+    [1]=>
+    string(1) "1"
+    [2]=>
+    string(1) "2"
+    [3]=>
+    string(1) "3"
+  }
+}
+array(1) {
+  [1]=>
+  array(1) {
+    [3]=>
+    string(1) "3"
+  }
+}