Add test for issue #40894, which is fixed in main branch

tests/webtbs/tw40894.pp

@@ -0,0 +1,15 @@
+program test;
+{$ZEROBASEDSTRINGS ON}
+{$H+} // make sure it is AnsiString, not ShortString
+{$R+}
+
+Var
+  I: Integer;
+  S: String;
+Begin
+  S := 'abc';
+  WriteLn(Length(S));
+  For I := High(S) Downto Low(S) Do
+    WriteLn(I, ' ---> ', S[I]);
+End.
+