Make arbitraryShortToken to also generate [] and {} characters.

That shouldn't break roundtrip tests, but currently does.

I hope it is due broken pretty-printers. See https://github.com/haskell/cabal/pull/7056 where refactoring to using parsec parser began for config files of `cabal-install`.

Start by locating arbitraryShortToken and the roundtrip tests, then read the parsec parser refactoring in PR #7056 for the cabal-install config path. Done means the generator covers [] and {} while the relevant roundtrip tests still pass.

Written by the indexing model from the issue text.