patch 8.2.1824: Vim9: variables at the script level escape their scope

Problem:    Vim9: variables at the script level escape their scope.
Solution:   When leaving a scope remove variables declared in it.

src/testdir/test_vim9_script.vim

@@ -2685,6 +2685,56 @@ def Run_Test_define_func_at_command_line()
   delete('Xdidcmd')
 enddef
 
+def Test_script_var_scope()
+  var lines =<< trim END
+      vim9script
+      if true
+        if true
+          var one = 'one'
+          echo one
+        endif
+        echo one
+      endif
+  END
+  CheckScriptFailure(lines, 'E121:', 7)
+
+  lines =<< trim END
+      vim9script
+      if true
+        if false
+          var one = 'one'
+          echo one
+        else
+          var one = 'one'
+          echo one
+        endif
+        echo one
+      endif
+  END
+  CheckScriptFailure(lines, 'E121:', 10)
+
+  lines =<< trim END
+      vim9script
+      while true
+        var one = 'one'
+        echo one
+        break
+      endwhile
+      echo one
+  END
+  CheckScriptFailure(lines, 'E121:', 7)
+
+  lines =<< trim END
+      vim9script
+      for i in range(1)
+        var one = 'one'
+        echo one
+      endfor
+      echo one
+  END
+  CheckScriptFailure(lines, 'E121:', 6)
+enddef
+
 " Keep this last, it messes up highlighting.
 def Test_substitute_cmd()
   new