Add Fourmolu as a formatting provider

Author: georgefst

package.json

@@ -84,7 +84,7 @@
         "languageServerHaskell.formattingProvider": {
           "scope": "resource",
           "type": "string",
-          "enum": ["brittany", "floskell", "ormolu", "none"],
+          "enum": ["brittany", "floskell", "fourmolu", "ormolu", "none"],
           "default": "brittany",
           "description": "The tool to use for formatting requests."
         },