Allow Unicode for Double Colons.

[mkohlhaas]

For those who are using Unicode.

[wenzel-hoffman]

@mkohlhaas It’s a duplicate of https://github.com/neovimhaskell/haskell-vim/pull/106 but the latter takes it to the next level and supports all unicode operators.

[wenzel-hoffman]

@mkohlhaas I decided to maintain my own fork with Unicode support since my merge request wasn’t merged since 2018. See https://github.com/wenzel-hoffman/haskell-with-unicode.vim