Fix haskell-nvim colors

[rbong]

The latest change to haskell-vim broke haskell-nvim.

From this comment:

here's the highlight groups i use daily

haskell vim highlight groups

hi! link haskelltype gruvboxyellow

hi! link haskellidentifier gruvboxgreen

hi! link haskellseparator gruvboxfg3
hi! link haskelldelimiter gruvboxfg3
hi! link haskelloperators gruvboxblue
hi! link haskellbacktick haskelloperators

hi! link haskellstatement gruvboxorange

hi! link haskellconditional gruvboxred
hi! link haskellkeyword gruvboxred
hi! link haskelllet gruvboxred
hi! link haskellwhere gruvboxred

hi! link haskelldefault gruvboxaqua
hi! link haskellbottom gruvboxaqua
hi! link haskellblockkeywords gruvboxaqua
hi! link haskellimportkeywords gruvboxaqua
hi! link haskelldeclkeyword gruvboxaqua
hi! link haskelldecl gruvboxaqua
hi! link haskellderiving gruvboxaqua
hi! link haskellassoctype gruvboxaqua

hi! link haskellnumber gruvboxpurple
hi! link haskellpragma gruvboxpurple

hi! link haskellstring gruvboxgreen
hi! link haskellchar gruvboxgreen

unfortunately, this looks nice with haskell-vim, but not great for the ftplugin that ships with vim. idk if there is precedence for supporting conflicting ftplugins like this, or if this would be better suited to being fixed upstream haskell-vim

[rbong]

Related: neovimhaskell/haskell-vim#123