Implement parsing gen fn

Generator functions were implemented in
rust-lang/rust#118457,
tracking issue rust-lang/rust#117078.

Author: narpfel

grammar.js

@@ -462,6 +462,7 @@ module.exports = grammar({
       'default',
       'const',
       'unsafe',
+      'gen',
       $.extern_modifier,
     )),
 

src/grammar.json

@@ -161,3 +161,27 @@ gen move {}
   (expression_statement
     (gen_block
       (block))))
+
+================================================================================
+gen fn
+================================================================================
+
+gen fn f() -> u8 {
+    yield 42;
+    yield;
+}
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (function_item
+    (function_modifiers)
+    (identifier)
+    (parameters)
+    (primitive_type)
+    (block
+      (expression_statement
+        (yield_expression
+          (integer_literal)))
+      (expression_statement
+        (yield_expression)))))