Add find, find_index, has, and reject filters to arrays

[karreiro]

This PR implements the RFC#1864 and introduces support for the find, find_index, has, and reject filters into the Liquid Array API.

The new filters support multi-level access to drops, so they can access properties like this:

{% assign product = products | find: 'price.unit', 'BRL' %}

For the sake of consistency, multi-level access has been introduced for all related filters in the Array API: map, sort, sort_natural, sum, uniq, where, and compact.

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Special thanks to @andershagbard who started this work on #1573 and #1749. I'm including those commits as part of this PR; while the implementation approach is slightly different, important work happened there, and it's valuable to have that context in the Liquid commit history.

Also, thank you to everyone who contributed with thoughts and suggestions in the RFC.