Add bigdecimal as a dependency

bigdecimal is going to move from being a default gem to a bundled
gem in Ruby 3.4, with warnings in Ruby 3.3.  bigdecimal has been
a default gem since Ruby 1.9, so this should be backwards compatible.

sequel.gemspec

@@ -23,6 +23,7 @@ SEQUEL_GEMSPEC = Gem::Specification.new do |s|
   s.require_path = "lib"
   s.bindir = 'bin'
   s.executables << 'sequel'
+  s.add_dependency "bigdecimal"
   s.add_development_dependency "minitest", '>=5.7.0'
   s.add_development_dependency "minitest-hooks"
   s.add_development_dependency "minitest-global_expectations"