feat(haskell): add roman numerals decoder kata

haskell/10_split_strings.hs

@@ -1,4 +1,4 @@
-module Codewars.Kata.SplitStrings where
+module Codewars.Kata.SplitStrings (solution) where
 
 import qualified Data.Char as Char
 

haskell/11_roman_numerals_decoder.hs

@@ -0,0 +1,23 @@
+{-# LANGUAGE NumericUnderscores #-}
+module Roman (solution) where
+
+solution :: String -> Int
+solution = sum . mergeParts . map fromRoman
+
+mergeParts :: [Int] -> [Int]
+mergeParts [] = []
+mergeParts [n1] = [n1]
+mergeParts (n1:n2:rest)
+  | n1 < n2 = n2 - n1 : mergeParts rest
+  | otherwise = n1 : mergeParts (n2:rest)
+
+fromRoman :: Char -> Int
+fromRoman c = case c of
+  'I' -> 1
+  'V' -> 5
+  'X' -> 10
+  'L' -> 50
+  'C' -> 100
+  'D' -> 500
+  'M' -> 1_000
+  _   -> 0