Feature: #95037 - New startingPoints setting for FormEngine treeConfig

See Issue #95037

Description

The TCA option treeConfig used in renderType=selectTree and type=category has a new setting startingPoints that allows to set multiple records as roots for tree records.

Impact

The setting takes a CSV value, e.g. 2,3,4711, which takes records of the pids 2, 3 and 4711 into account and creates a tree of these records.

Additionally, each value used in startingPoints may be fed from a site configuration by using the ###SITE:### syntax.

Example:

# Site config
base: /
rootPageId: 1
categories:
   root: 123
// Example TCA config
'config' => [
    'treeConfig' => [
        'startingPoints' => '1,2,###SITE:categories.root###',
    ],
],

This will evaluate to 'startingPoints' => '1,2,123'.