patch 8.2.1079: Vim9: no line break allowed in a while loop

Problem:    Vim9: no line break allowed in a while loop.
Solution:   Update stored loop lines when finding line breaks.

src/testdir/test_vim9_cmd.vim

@@ -131,12 +131,29 @@ def Test_if_linebreak()
 enddef
 
 def Test_while_linebreak()
-  " TODO: line break in :while expression doesn't work yet
   let lines =<< trim END
       vim9script
       let nr = 0
-      while nr < 10 + 3
-            nr = nr + 4
+      while nr <
+              10 + 3
+            nr = nr
+                  + 4
+      endwhile
+      assert_equal(16, nr)
+  END
+  CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      let nr = 0
+      while nr
+            <
+              10
+              +
+              3
+            nr = nr
+                  +
+                  4
       endwhile
       assert_equal(16, nr)
   END