Add base64 as runtime dependency

[sawirricardo]

Problem

Ruby 3.3+ no longer includes base64 as a default gem. Loading liquid on Ruby 3.3+ produces a deprecation warning:

liquid-5.4.0/lib/liquid/standardfilters.rb:4: warning: base64 was loaded from the standard library,
but will no longer be part of the default gems since Ruby 3.4.0.
Add base64 to your Gemfile or gemspec.

Solution

Add base64 as an explicit runtime dependency in the gemspec, matching the approach already used for bigdecimal and strscan.

Testing

All 774 existing tests pass on Ruby 3.4.7.

Fixes #1772