[fix] Emit at_offset for anonymous block args (#944)

* Fixtures

* Emit `at_offset` for anonymous block args

Author: reese

fixtures/small/anonymous_blockarg_actual.rb

@@ -1,3 +1,25 @@
 def foo(a, b, &)
   bar a, b, &
 end
+
+def foo(
+  a,
+  &
+)
+  bar(
+    a,
+  &)
+end
+
+def foo(
+  a,
+  *b,
+  &
+)
+bar(
+    a,
+    b,
+    &
+  )
+end
+

fixtures/small/anonymous_blockarg_expected.rb

@@ -1,3 +1,25 @@
 def foo(a, b, &)
   bar(a, b, &)
 end
+
+def foo(
+  a,
+  &
+)
+  bar(
+    a,
+    &
+  )
+end
+
+def foo(
+  a,
+  *b,
+  &
+)
+  bar(
+    a,
+    b,
+    &
+  )
+end

librubyfmt/src/format_prism.rs

@@ -1636,6 +1636,8 @@ fn format_block_parameter_node<'src>(
             let ident_str = ident.as_slice();
             ps.bind_variable(ident_str);
             handle_string_at_offset(ps, ident_str, block_arg.name_loc().unwrap().end_offset());
+        } else {
+            ps.at_offset(block_arg.location().start_offset());
         }
     });
 }