[5.x] Extend get modifier

[marcorieser]

This PR extends the existing get modifier and:

• Adds Tests for existing get modifier logic (separate commit)
• Adds support for a specific index when modifying an array (e.g. entries_as_array | get(1) or address_as_array | get('street'))
• Adds support for Collections (same behavior as for arrays)
• Adds support for Query Builders. They get resolved and return the whole result or (if given) a specific index, like for arrays