Add prompt_suggestion icon to list of RTL mirroring icons. (#191741)

Mark the `prompt_suggestion` icon as to be mirrored in RTL languages so
that it points into the text appropriately.

Before:
<img width="161" height="59" alt="Screenshot 2026-08-25 at 12 46 05 PM"
src="https://github.com/user-attachments/assets/2033efec-6707-415b-98ec-85e5acb43d35"
/>

After:
<img width="159" height="57" alt="Screenshot 2026-08-25 at 12 46 12 PM"
src="https://github.com/user-attachments/assets/c2834ac6-4cca-4fe7-be83-ba91254e972f"
/>

Fixes https://github.com/flutter/flutter/issues/191742

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