Add QuillFocusNode with support for keyboard enable/disable while preserving focus

[ppamorim]

Description

This PR introduces the ability to suppress the on-screen keyboard when a user places the cursor in the editor, while preserving the focus state. This feature is particularly useful for scenarios where speech-to-text (STT) is the primary input method and the presence of the keyboard obstructs UI elements or interferes with dictation.

To achieve this:

• A new QuillFocusNode subclass was created with a keyboardEnabled toggle.
• The logic inside openConnectionIfNeeded() in raw_editor_state_text_input_client_mixin.dart has been adjusted to respect this toggle.
• If keyboardEnabled is set to false, the editor will still receive focus without triggering the software keyboard.

This allows developers to implement an "edit mode" (enabled by default on the demo) or similar UX patterns where the keyboard only appears when explicitly desired by the user.

Related Issues

• Related to Add Option to Disable On-Screen Keyboard When Cursor Is Placed Manually singerdmx/flutter-quill#2587

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.