fix: the composing text did not show an underline during IME conversion

[agata]

Description

Flutter Quill did not previously reflect the composing range during IME conversion in the UI. I have now added an underline beneath the text being composed based on changes to the composing range from the IME. This aligns the behavior with Flutter's TextFormField.

Users who rely on IME will now have visual feedback for the currently composing text, improving the user experience. (For IME users, the lack of this support previously resulted in a significantly poor user experience, as it’s crucial for them.)

I have tested this with the Japanese IME on iOS, macOS, and Windows. I haven’t tested it with other languages, but I believe it should work fine. If anyone is using an IME for other languages, I would appreciate your feedback.

Before:

quill_editor.mp4

After:

default.mp4

Related Issues

• Fix No underline displayed for characters being converted with IME on iOS #2179

Type of Change

• ✨ New feature: Adds new functionality without breaking existing features.
• 🛠️ Bug fix: Resolves an issue without altering current behavior.
• 🧹 Code refactor: Code restructuring that does not affect behavior.
• ❌ Breaking change: Alters existing functionality and requires updates.
• 🧪 Tests: Adds new tests or modifies existing tests.
• 📝 Documentation: Updates or additions to documentation.
• 🗑️ Chore: Routine tasks, or maintenance.
• ✅ Build configuration change: Changes to build or deploy processes.

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