patch 8.1.2262: unpack assignment in function not recognized

Problem:    Unpack assignment in function not recognized.
Solution:   Skip over "[a, b]". (closes #5051)
diff --git a/src/testdir/test_let.vim b/src/testdir/test_let.vim
index 64d178c..76b7505 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -303,4 +303,12 @@
      END
   endif
   call assert_equal([], check)
+
+  " unpack assignment
+  let [a, b, c] =<< END
+     x
+     \y
+     z
+END
+  call assert_equal(['     x', '     \y', '     z'], [a, b, c])
 endfunc