Add base64 dependency for ruby >= 3.4.x

[Samsinite]

Step To Reproduce / Observed behavior

Install ruby v3.4.x Install playwright-ruby-client Try to load playwright require 'playwright Thankfully ruby yells about the missing dependency in a clear way:

warning: base64 was loaded from the standard library, but is not part of the default gems starting from Ruby 3.4.0.
You can add base64 to your Gemfile or gemspec to silence this warning.

Expected behavior

The dependency is correctly specified for those on Ruby >= v3.4.x and the library loads when required.

Environment

ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [arm64-darwin24]