support variance annotation

Author: aspeddro

grammar.js

@@ -257,7 +257,12 @@ module.exports = grammar({
 
     type_parameters: $ => seq(
       '<',
-      commaSep1t($.type_identifier),
+      commaSep1t(
+        seq(
+          optional(choice('+', '-')),
+          $.type_identifier
+        )
+      ),
       '>',
     ),
 

test/corpus/type_declarations.txt

@@ -417,3 +417,20 @@ type test = (. ~attr: string) => unit
             (type_annotation
               (type_identifier)))))
       (unit_type))))
+
+===========================================
+Variance annotations
+===========================================
+
+type t<+'a>
+type t<-'a>
+
+---
+
+(source_file
+  (type_declaration
+    (type_identifier)
+    (type_parameters (type_identifier)))
+  (type_declaration
+    (type_identifier)
+    (type_parameters (type_identifier))))