Ternary operator feature support

I know this feature has been requested before in this thread: https://github.com/Shopify/liquid/issues/236

And based on 85 liking the idea, versus 2 not liking the idea says something.

But that issue was closed back in 2016, and as the docs says:

Things we will merge
- Features that are likely to be useful to the majority of Liquid users

I personally still see a great advantage in having the opportunity to use ternary operators, the code is way more slick and friendly to look at.

I saw the default filter was suggested, but doesn't always come in handy, when using logic.

Without ternary

{%- liquid
   assign has_selected_variant = false

   if product.selected_variant != nil
      assign has_selected_variant = true
   endif
-%}

With ternary

{%- assign has_selected_variant = product.selected_variant != nil ? true : false -%}

Start by reading the prior request in issue #236 and compare the provided “without ternary” and “with ternary” examples against current Liquid behavior. Define the supported syntax and acceptance tests before implementation; done means the ternary form works without breaking existing Liquid logic.