Code block cursor confusion problem when `backspace`

[Jocs]

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

• [x] Confirmed that it can be reproduced in muya.

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Describe the bug If the backspace is preceded by a newline character, it will result in an error.

To Reproduce Steps to reproduce the behavior:

1. Create javascript code block
2. Input codes bellow

function add(){

}|

4. The cursor is at the end of function add
5. Press backspace

Expected behavior Input normally.

Actual behavior The cursor flies inside the topmost code

Operation System and Browser version (please complete the following information):

• OS: [e.g. macOS 13.5 (22G74)]
• Browser: firefox only

Muya Version e.g: 0.10.11