fix: prevent toolbar flicker when tapping a checklist checkbox

[Alspb]

Description

Fixes a brief toolbar flicker when tapping a checklist checkbox, and retires the internal QuillController.toolbarButtonToggler workaround by deprecating it rather than removing it.

Current behavior: Tapping a checklist checkbox also fires the editor's tap gesture (onSingleTapUp → selectPosition), which moves the caret to the tapped line for one frame. The toolbar briefly reflects that line's style — the header, inline, and color buttons flicker — before _handleCheckboxTap's post-frame restore puts the selection back. The checklist button additionally flashes because formatText stages the checked/unchecked attribute into toggledStyle, which getSelectionStyle() merges.

New behavior: The gesture-driven selection change from the checkbox tap is ignored (_ignoreCheckboxTapSelectionChange), so the caret — and the toolbar — stay put. The checkbox is formatted silently (shouldNotifyListeners: false) so the staged toggledStyle doesn't momentarily light up the checklist button; the
existing post-frame updateSelection emits a single clean notification.

This removes the need for the old toolbarButtonToggler workaround, so its consumers (header buttons/dropdown, checklist button, line-height dropdown) are removed.
The field itself is public but undocumented and internal-only; instead of hard-removing it (a breaking change in a minor release), it's kept as a @Deprecated, @internal no-op and scheduled for removal in a future version.

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.

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