highlight groups not matching vim OotB ftplugin cause themes to break when switching between haskell-vim and the default ftplugin

[mlopes]

As per the issue happening here with gruvbox:

https://github.com/gruvbox-community/gruvbox/pull/87#issuecomment-762871866

The fact that the groups don't match means that themes will either look good with haskell-vim or with vim's out of the box haskell support.