patch 8.2.2157: Vim9: can delete a Vim9 script variable from a function

Problem:    Vim9: can delete a Vim9 script variable from a function.
Solution:   Check the variable is defined in Vim9 script. (closes #7483)
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index 60c8fe9..ad7b85b 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -1211,6 +1211,14 @@
   CheckScriptFailure([
    'vim9script',
    'var svar = 123',
+   'func Func()',
+   '  unlet s:svar',
+   'endfunc',
+   'Func()',
+   ], 'E1081:')
+  CheckScriptFailure([
+   'vim9script',
+   'var svar = 123',
    'def Func()',
    '  unlet s:svar',
    'enddef',