Made flex and flexChildren available again

[js-goupil]

Background

Our Stack implementation on POS still has some limitations when it comes to avoiding flex and flexChildren. For example, we don't have any way to implement JustifyItems right now which would let the buttons and other components stretch across the main axis, so we still need flexChildren to accomplish this. As for flex, we need to expose this property so that partners can tell the Stack to take the whole screen if they want. Ideally in the future we refactor our Screen implementation to be flex independent, but this will take time and work that we don't have the capacity for right now.

Solution

Removes the deprecation warnings for flex and flexChildren. It also fixes up some documentation.

🎩

• ...

Checklist

• I have 🎩'd these changes
• I have updated relevant documentation

[github-actions]

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