ZStack: support RTL layouts when applying offset

[ciampo]

Description

The ZStack component has an offset prop which establishes the amound of offset between each child on the X axis.

This PR flips the value of the offset when the layout switches from LRT to RTL, so that the original intent of the offset value is preserved.

How has this been tested?

Open Storybook, check the ZStack and ItemGroup stories

Screenshots

Before

zstack-before.mp4

After

zstack-after.mp4

Types of changes

Fix (RTL support)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• N/A I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[mirka]

Nice improvement 🚢