patch 8.2.0306: Vim9: :substitute(pat(repl does not work in Vim9 script

Problem:    Vim9: :substitute(pat(repl does not work in Vim9 script.
Solution:   Remember starting with a colon. (closes #5676)

src/testdir/test_vim9_script.vim

@@ -580,6 +580,21 @@ def Test_substitute_cmd()
   setline(1, 'something')
   :substitute(some(other(
   assert_equal('otherthing', getline(1))
+  bwipe!
+
+  " also when the context is Vim9 script
+  let lines =<< trim END
+    vim9script
+    new
+    setline(1, 'something')
+    :substitute(some(other(
+    assert_equal('otherthing', getline(1))
+    bwipe!
+  END
+  writefile(lines, 'Xvim9lines')
+  source Xvim9lines
+
+  delete('Xvim9lines')
 enddef