Extend support for outlining in Gleam (#6834)

This PR extends support for outlines in the recently-added Gleam
support.

Note that I'm leaving the release notes section blank as we haven't yet
released Gleam support, so it can just roll up into that item.

Release Notes:

- N/A

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

@@ -1,4 +1,31 @@
+(external_type
+    (visibility_modifier)? @context
+    "type" @context
+    (type_name) @name) @item
+
+(type_definition
+    (visibility_modifier)? @context
+    (opacity_modifier)? @context
+    "type" @context
+    (type_name) @name) @item
+
+(data_constructor
+    (constructor_name) @name) @item
+
+(data_constructor_argument
+    (label) @name) @item
+
+(type_alias
+    (visibility_modifier)? @context
+    "type" @context
+    (type_name) @name) @item
+
 (function
     (visibility_modifier)? @context
     "fn" @context
     name: (_) @name) @item
+
+(constant
+    (visibility_modifier)? @context
+    "const" @context
+    name: (_) @name) @item