build: add missed default-language field to shellcheck-dev

> Warning: Packages using 'cabal-version: >= 1.10' and before
> 'cabal-version: 3.4' must specify the 'default-language' field for
> each component (e.g. Haskell98 or Haskell2010). If a component uses
> different languages in different modules then list the other ones in
> the 'other-languages' field.