Copy-paste of task list item and subitems only copies the item

[jpmv27]

• [x] Searched existing issues to avoid creating duplicates.

• [x] Confirmed that it can be reproduced in built-in themes without customized css.

• [x] Searched http://support.typora.io/

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Describe the bug Copy-paste of task list item and sub-items only copies the item

To Reproduce Steps to reproduce the behavior:

1. Create a task list with items and sub-items (2nd level items)
2. Select an item along with its sub-items
3. Copy (e.g., ^C)
4. Paste elsewhere (e.g., ^V)

Expected behavior The copied item and sub-items are pasted

Actual behavior Only the item is pasted, without the sub-items

Desktop (please complete the following information):

• OS: Windows 10 Pro (Version 10.0.19045 Build 19045)

Typora Version e.g: 1.9.5

Additional context N/A