feat: add mergeImeCompositionHistory config to group IME intermediate states in undo stack

[nandin9]

Description

Currently, when users type using an Input Method Editor (IME) like Chinese Pinyin or Japanese, every intermediate keystroke (composing state) is recorded as a separate step in the History undo stack. This leads to a broken undo/redo experience where pressing Cmd+Z or Ctrl+Z undoes individual pinyin letters instead of the final committed word.

This PR introduces a new configurable feature to group these IME composition states into a single, clean undo step.

Changes Made

• Added mergeImeCompositionHistory in QuillEditorConfig: A new boolean flag (defaulting to true) allowing developers to control this IME history merging behavior.
• Added forceMergeNext in History: A new setter in the bottom-layer History class. It utilizes Dart's Future.microtask to automatically and safely reset its state after the current synchronous event loop, ensuring no state leakage to subsequent regular key presses.
• Intercepted updateEditingValue in RawEditorState: Reads the isComposing state from TextEditingValue and flags the history stack to merge the upcoming delta if the previous state was composing.

Visual Proof (Before & After)

• Before: Pressing Undo removes intermediate pinyin characters one by one.

video-1774413039976.mp4

• After: Pressing Undo cleanly removes the entire committed phrase as a single logical step.

video-1774412915619.mp4

Type of Change

• ✨ Feature: New functionality without breaking existing features.
• 🛠️ Bug fix: Resolves an issue without altering current behavior.
• 🧹 Refactor: Code reorganization, no behavior change.
• ❌ Breaking: Alters existing functionality and requires updates.
• 🧪 Tests: New or modified tests
• 📝 Documentation: Updates or additions to documentation.
• 🗑️ Chore: Routine tasks, or maintenance.
• ✅ Build configuration change: Build/configuration changes.