Move module methods to `Liquid2::Environment` instance methods

[jg-rp]

Move the following methods of the Liquid2 module to be instance methods of Liquid2::Environment.

• Liquid2.truthy?
• Liquid2.eq?
• Liquid2.lt?
• Liquid2.contains?
• Liquid2.to_liquid_string alias Liquid2.to_s
• Liquid2.to_output_string alias Liquid2.to_output_s
• Liquid2.to_liquid_int alias Liquid2.to_i

Then, Environment subclasses can override these methods to customize template rendering behaviour.