[FEATURE] Add f:flatten ViewHelper

[pixeldesu]

This commit adds the f:flatten ViewHelper, allowing to flatten multi-dimensional array structures.

Example

Input:

[[1, 2], [3, 4]]

<f:flatten value="{0: {0: '1', 1: '2'}, 1: {0: '3', 1: '4'}}" />

Output:

{0: '1', 1: '2', 2: '3', 3: '4'}

[s2b]

Thank you for your contribution!