Haskell: v3 config.json stub

This stub contains all things necessary but the list of exercises.

The example C# config.json did not contain the fields `ignore_pattern`
and `solution_pattern` present in Haskell's config.json, but the
[migration guide][1] also did not speak of deprecating any properties.

For this reason, this PR preserves those properties and adds new ones.

[1]: https://github.com/exercism/v3/blob/master/docs/maintainers/migrating-your-config-json-files.md

Author: sshine

languages/haskell/config.json

@@ -0,0 +1,16 @@
+{
+  "language": "Elixir",
+  "version": 3,
+  "active": true,
+  "blurb": "Haskell is a functional programming language which is pure and statically-typed. It's known for lazy evaluation, where evaluation is deferred until necessary, and its purity, where monads are used for working with side-effects.",
+  "online_editor": {
+    "indent_style": "space",
+    "indent_size": 2
+  },
+  "ignore_pattern": "example",
+  "solution_pattern": "example.*[.]hs",
+  "exercises": {
+    "concept": [],
+    "practice": []
+  }
+}