Compatibility with Ractor

[larskanis]

ruby-pg is currently not marked as Ractor-safe. That means that it isn't usable in Ractors other than the main Ractor. The requirements for Ractor-safety are defined in https://github.com/ruby/ruby/blob/f51a6ace06cecd193bf00747a63809849e254050/doc/extension.rdoc#appendix-f-ractor-support-

Ractor-safety should be checked and if/when it's ensured, it can be declared like here: https://github.com/ruby/io-wait/commit/ba338b4764