Comment tag parsing

[jg-rp]

Shopify/liquid have changed comment tag parsing behaviour. Other than {% raw %} tags, all valid and invalid tags inside a comment block are now not parsed, whereas before they were, potentially causing errors from ill formed tags inside the comment block.

This new behaviour is close to Python Liquid’s default behaviour, with the exception that Shopify/Liquid now allows any number of fully-formed nested comment blocks, which causes a LiquidSyntaxError in Python Liquid.

We should include an implementation of the comment tag that matches Shopify/liquid and use it as the default in liquid.future.Environment.