DropdownMenu v2: Fix submenu chevron direction in RTL languages

[walbo]

What?

Make sure the RTL styles are beeing applied for the submenu trigger item.

Why?

The arrows should point the other way for RTL languages.

How?

Fix how RTL styles are added.

Testing Instructions

1. Open up storybook and navigate to the dropdown v2 component.
2. Enable RTL
3. Confirm the arrows point to the left on the submenu trigger item.

Screenshots or screencast

Before	After

[github-actions]

Flaky tests detected in 392b882.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/6015849821
📝 Reported issues:

• [Flaky Test] Should load the view script if the lightbox is disabled in theme.json but enabled via block settings #53782 in /test/e2e/specs/editor/various/behaviors.spec.js

[apeatling]

Confirmed this works both in LTR and RTL. LGTM!

[ciampo]

LGTM 🚀

Thank you, @walbo (good to see you around again!)

Interesting that our static linting tools didn't pick up the error.