Modify the `replace_and_mark_text_in_range` method in the `Editor` to
keep the cursor at the start of the preedit range during IME
composition. Previously, the cursor would move to the end of the preedit
text with each update, causing the IME candidate box to shift (e.g.,
when typing pinyin with Fcitx5 on Wayland). This change ensures the
cursor and candidate box remain fixed until the composition is
committed, improving the IME experience.

Closes #21004

Release Notes:

- N/A

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Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>
Co-authored-by: 张小白 <364772080@qq.com>