[ADD] ability to add placeholder textstyle

[hardiklakhalani]

Description

This PR adds support for customizing the placeholder text style (including font size) in the Flutter Quill editor. Previously, the placeholder text style was hardcoded with a fontSize of 20 and could not be customized by users.

Current Behavior:

• Placeholder text uses a fixed style with fontSize: 20 defined in DefaultStyles.getInstance()
• Users cannot customize placeholder appearance (font size, color, etc.)
• The only way to change placeholder style was to override the entire DefaultStyles via customStyles, which is cumbersome

New Behavior:

• Added optional placeholderStyle parameter to QuillEditorConfig and QuillRawEditorConfig
• Users can now pass a TextStyle to customize placeholder appearance (fontSize, color, fontStyle, etc.)
• The provided placeholderStyle is merged with the default placeholder style, allowing partial customization
• Fully backward compatible - existing code continues to work without changes

Motivation:
Many users need to customize placeholder text appearance to match their app's design system. Currently, this requires creating a complete DefaultStyles override just to change the placeholder font size, which is overly complex for such a common use case.

Related Issues

• Related to placeholder customization requests
• Addresses the need for easier placeholder styling without full DefaultStyles override

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.

Changes Made

Files Modified

1. lib/src/editor/config/editor_config.dart

   • Added placeholderStyle parameter (optional TextStyle?) to QuillEditorConfig
   • Added documentation for the new parameter
   • Updated copyWith() method to include placeholderStyle

2. lib/src/editor/raw_editor/config/raw_editor_config.dart

   • Added placeholderStyle parameter (optional TextStyle?) to QuillRawEditorConfig
   • Added documentation for the new parameter

3. lib/src/editor/editor.dart

   • Updated to pass placeholderStyle from QuillEditorConfig to QuillRawEditorConfig

4. lib/src/editor/raw_editor/raw_editor_state.dart

   • Added logic in didChangeDependencies() to merge custom placeholderStyle with default placeholder style
   • Uses DefaultStyles.merge() to update only the placeholder style

Usage Example

QuillEditor.basic(
  controller: quillController,
  config: QuillEditorConfig(
    placeholder: 'Write a message...',
    placeholderStyle: TextStyle(
      fontSize: 14,  // Custom font size
      color: Colors.grey.shade600,
      fontStyle: FontStyle.italic,
    ),
  ),
)

Testing

• ✅ Verified backward compatibility (works without placeholderStyle)
• ✅ Tested with custom fontSize only
• ✅ Tested with multiple style properties (fontSize, color, fontStyle)
• ✅ Verified that default placeholder style is preserved when not overridden
• ✅ Confirmed existing placeholder functionality remains intact

Backward Compatibility

✅ Fully backward compatible

• placeholderStyle is optional (nullable)
• If not provided, default placeholder style (fontSize: 20) is used
• Existing code requires no changes