Feature request: support UnicodeSyntax extension

[unclechu]

Highlighting unicode operators as other operators would be good.
This PR solves it.

[unclechu]

I decided to maintain my own fork since the status quo doesn’t seem to change any time soon (the merge request was created in 2018).

The fork with Unicode support: https://github.com/wenzel-hoffman/haskell-with-unicode.vim

Closing this request.