feat: Extern types

Tracking issue: rust-lang/rust#43467
I am not sure if the parsing as an associated type is really ideal,
but introducing a specific node type would likely introduce more
conflicts.

Author: wetneb

grammar.js

@@ -519,6 +519,7 @@ module.exports = grammar({
     ),
 
     associated_type: $ => seq(
+      optional($.visibility_modifier),
       'type',
       field('name', $._type_identifier),
       field('type_parameters', optional($.type_parameters)),

test/corpus/declarations.txt

@@ -2091,6 +2091,24 @@ where
                 (type_identifier)))))))
     (block)))
 
+================================================================================
+Extern types
+================================================================================
+
+extern {
+    pub type Opaque;
+}
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (foreign_mod_item
+    (extern_modifier)
+    (declaration_list
+      (associated_type
+        (visibility_modifier)
+        (type_identifier)))))
+
 ================================================================================
 Higher-ranked types
 ================================================================================