patch 8.2.1657: Vim9: no proper error for nested ":def!"

Problem:    Vim9: no proper error for nested ":def!".
Solution:   Check for "!". (closes #6920)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 4e79c89..e0b43ae 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -134,6 +134,23 @@
   CheckDefFailure(['func Nested()', 'endfunc'], 'E1086:')
   CheckDefFailure(['def s:Nested()', 'enddef'], 'E1075:')
   CheckDefFailure(['def b:Nested()', 'enddef'], 'E1075:')
+
+  CheckDefFailure([
+        'def Outer()',
+        '  def Inner()',
+        '    # comment',
+        '  enddef',
+        '  def Inner()',
+        '  enddef',
+        'enddef'], 'E1073:')
+  CheckDefFailure([
+        'def Outer()',
+        '  def Inner()',
+        '    # comment',
+        '  enddef',
+        '  def! Inner()',
+        '  enddef',
+        'enddef'], 'E1117:')
 enddef
 
 func Test_call_default_args_from_func()