patch 8.2.1718: Vim9: :def function disallows "firstline" for no good reason

Problem:    Vim9: :def function disallows "firstline" and "lastline" argument
            names for no good reason.
Solution:   Don't check the arguments for a :def function. (closes #6986)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index fe7bf69..4051650 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -486,6 +486,15 @@
   CheckScriptFailure(['def Func(arg: number)', 'arg = 3', 'enddef', 'defcompile'], 'E1090:')
 enddef
 
+" These argument names are reserved in legacy functions.
+def WithReservedNames(firstline: string, lastline: string): string
+  return firstline .. lastline
+enddef
+
+def Test_argument_names()
+  assert_equal('OK', WithReservedNames('O', 'K'))
+enddef
+
 def Test_call_func_defined_later()
   g:DefinedLater('one')->assert_equal('one')
   assert_fails('NotDefined("one")', 'E117:', '', 2, 'Test_call_func_defined_later')