patch 8.2.2256: Vim9: cannot use function( after line break in :def function

Problem:    Vim9: cannot use function( after line break in :def function.
Solution:   Check for "(" after "function". (closes #7581)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index bdced02..d0d4e3b 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -260,6 +260,11 @@
   CheckScriptSuccess(lines)
 enddef
 
+def Test_not_nested_function()
+  echo printf('%d',
+      function('len')('xxx'))
+enddef
+
 func Test_call_default_args_from_func()
   call MyDefaultArgs()->assert_equal('string')
   call MyDefaultArgs('one')->assert_equal('one')