Add support for arbitrary expressions as elements of array

[navrocky]

This MR adds support for parsing and executing expressions in arrays.

These examples show how the improvements work:

// Array literals with arbitrary expressions as elements
render("{% set neighbour=\"Peter\" %} {{ [neighbour, \"Anna\"] }}", data); // "[\"Peter\",\"Anna\"]"

// Iterating over an inline array containing variables
render("{% for guest in [neighbour, \"Anna\"] %}{{ guest }} {% endfor %}", data); // "Peter Anna "