Add where/reject option to for

Hi,

In Shopify, we often need to loop through blocks and ignore some depending on conditions. It would be very handy if for loops would support an extra "where" and "reject" filters:

{%- for block in section.blocks where: 'settings.image' -%}
  Only loop through blocks whose image is defined 
{%- else -%}
  Show placeholder
{%- endfor -%}

This would make the syntax much more expressive and would remove a lot of useless creation of temporary variables (which are often source of hard to track bug when they override already existing variables).

Start by locating Liquid's existing for-loop implementation and tests, then compare how the proposed where filter would interact with section.blocks and the existing else branch. Define the behavior and syntax for both where and reject, including the settings.image example, and consider the feature complete when filtered loops and empty-result handling are covered by tests.