Add Fourmolu config property (#736)

Author: georgefst

package.json

@@ -262,6 +262,12 @@
           "scope": "resource",
           "type": "boolean"
         },
+        "haskell.plugin.fourmolu.config.external": {
+          "default": false,
+          "markdownDescription": "Call out to an external \"fourmolu\" executable, rather than using the bundled library",
+          "scope": "resource",
+          "type": "boolean"
+        },
         "haskell.plugin.gadt.globalOn": {
           "default": true,
           "description": "Enables gadt plugin",