root: Add window_shadow_size option.

[richerfu]

Description

Recently, i made an adaption for gpui that can run in OpenHarmony/HarmonyNext. And i use gpui and gpui-component to make some applications. I find the soft keyboard will effect the dialog mask offset.

Is it possiable for us to add some cfg feature for ohos? Thanks.

Screenshot

Keyboard hide:

Keyboard show:

Break Changes

• No breaking

How to Test

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)