Fix flex-child ordering

[istarkov]

Description

closes #1833

Steps for reproduction

Create Flex box, add 2 boxes
Change order on any box

Code Review

• hi @kof, I need you to do
  • conceptual review (architecture, feature-correctness)
  • detailed review (read every line)
  • test it on preview

Before requesting a review

• made a self-review
• added inline comments where things may be not obvious (the "why", not "what")

Before merging

• tested locally and on preview environment (preview dev login: 5de6)
• updated test cases document
• added tests
• if any new env variables are added, added them to .env.example and the builder/env-check.js if mandatory

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Name	Status	Preview	Updated (UTC)
webstudio-builder	✅ Ready (Inspect)	Visit Preview	Jun 23, 2023 6:48am