GHC-58481 overloaded labels example

[endgame]

The absence of Overloaded Labels can cause confusing parse errors because # is lexed as an operator, which then confuses the parser. Try to help the situation by adding an example.

Note: I had to bypass pre-commit hooks when committing the example, because ormolu wanted to format the "before" example and failed because it is designed not to parse. Is there a way to exclude message-index/messages/GHC-58481/example5/before/Label.hs from autoformatting?