Add Haskell buffer symbol search (#7331)

This PR is a follow-up from
https://github.com/zed-industries/zed/pull/6786#issuecomment-1912912550
and adds an `outline.scm` file for buffer symbol search support in
Haskell.

Release Notes:

- Added buffer symbol search support for Haskell

crates/zed/src/languages/haskell/outline.scm

@@ -0,0 +1,26 @@
+(adt
+  "data" @context
+  name: (type) @name) @item
+
+(type_alias
+  "type" @context
+  name: (type) @name) @item
+
+(newtype
+  "newtype" @context
+  name: (type) @name) @item
+
+(signature
+  name: (variable) @name) @item
+
+(class
+  "class" @context
+  (class_head) @name) @item
+
+(instance
+  "instance" @context
+  (instance_head) @name) @item
+
+(foreign_import
+  "foreign" @context
+  (impent) @name) @item