Possible addition of a "basic" engine without the default filters and tags

[jam3sn]

Checklist

• [ x ] I have searched the issue list
• I have tested my example against Shopify Liquid. (This isn't necessary if the actual behavior is a panic, or an error for which IsTemplateError returns false.)

Expected Behavior

Currently creating an instance of the Liquid Engine registers the various default tags, blocks and filters. I'd like to propose a "basic" variation of the engine which doesn't include these defaults.

Actual Behavior

Detailed Description

My use case is a limited liquid template that users can write some basic markup with access to a selection of variables (bindings) and a few custom filters, but nothing more. I attempted this creating my own instance of liquid.Engine{}, however found that if the template contained say a conditional block, an error would occur because Grammar is nil, and I can't use an instance of liquid.Config in liquid.Engine because cfg is private.

Here's a Go Playground of this in action: https://go.dev/play/p/VK98dDtODp4

Possible Solution

// NewBasicEngine returns a new Engine.
func NewBasicEngine() *Engine {
	reutrn &Engine{render.NewConfig()}
}

If this is something you are open to, I'd be happy to submit a PR for review. Or perhaps if you have another idea or solution that would be fantastic.

[jam3sn]

Merged in #104