Add dependency bigdecimal

Avoid Ruby warning:
/gems/3.3.0+0/gems/bake-0.18.2/lib/bake/types/decimal.rb:25:
warning: bigdecimal will be not part of the default gems since Ruby
3.4.0. Add bigdecimal to your gems.rb. Also contact author of  to add
bigdecimal into its gemspec.

bake.gemspec

@@ -26,6 +26,7 @@ Gem::Specification.new do |spec|
 	spec.required_ruby_version = ">= 2.7.6b"
 
 	spec.add_dependency "samovar", "~> 2.1"
+	spec.add_dependency "bigdecimal"
 
 	spec.add_development_dependency "bundler"
 	spec.add_development_dependency "covered"