Improve Haskell syntax highlighting

[Andriamanitra]

Various improvements and fixes to the Haskell syntax:

• Better highlighting of operators (including user-defined ones)
• Highlight SomeType and T.SomeType as type but T.someFunction using default color. Capitalized identifiers in Haskell can be module names, type classes, types, or type/data constructors. Micro's syntax highlighting is not powerful enough to properly distinguish between these but I found using type for all of them a reasonable compromise.
• Add highlighting for unit instance (()) and numeric literals (int/float/octal/hexadecimal)
• Improve highlighting for escape sequences in strings
• Remove as from the list of keywords because it only works like a keyword in the context of import ... as ..., and it is very commonly used as an identifier
• Add highlighting for infix function application (eg. 5 `div` 3)
• Add highlighting for SemiGroup / Applicative / Monoid type classes