add built-in filter to flatten a nested array

[derkork]

When you do json-paths with foo[*].* you get nested arrays. while this is okay, sometimes you need to unwrap these into a single array. it would therefore be nice to have a function that would flatten one level of arrays in an array

{% let stuff = [[1,2,3], 4, [5,6,[7,8]] %}
{{ stuff | flatten }} {# prints [1,2,3,4,5,6,[7,8]] #}
{{ stuff | flatten | flatten }} {# prints [1,2,3,4,5,6,7,8] #}

Maybe also add a flatten_all filter which flattens until the array no longer contains any other arrays.