Add copyWith methods to DefaultStyles, HorizontalSpacing, and VerticalSpacing

[md-rifatkhan]

• ✨ 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.

Easily override style:

DefaultStyles _buildDefaults(BuildContext context) {
    // flutter quill
    final base = DefaultStyles.getInstance(context);
    final h1 = base.h1;

    return base.merge(
      DefaultStyles(
        // Before (Copied from default_styles.dart file)
        // h1: DefaultTextBlockStyle(
        //   TextStyle(
        //     fontSize: 34,
        //     color: Colors.white,
        //     letterSpacing: -0.5,
        //     height: 1.083,
        //     fontWeight: FontWeight.bold,
        //     decoration: TextDecoration.none,
        //   ),
        //   HorizontalSpacing(0, 0),
        //   const VerticalSpacing(16, 0),
        //   VerticalSpacing.zero,
        //   null,
        // ),

        // after (Modified)
        // h1: h1?.copyWith(
        //   style: h1.style.copyWith(color: Colors.blue),
        //   verticalSpacing: h1.verticalSpacing.copyWith(bottom: 20),
        // ),


      ),
    );
  }

[EchoEllet]

A minor issue: copyWith does not allow setting properties to null; attempting to do so will leave the property unchanged. There is a workaround, but before using it, we need to check whether there is a nullable or if there is a need to set it to null.

horizontalSpacing.copyWith(
  left: null // left will not set to null even if `this.left` type is nullable. 
)

The workaround is to wrap left in the copyWith inside a class, but this workaround should be avoided if not needed.

[EchoEllet]

Could you update CHANGELOG.md? Any modification is enough, so I can update update the file with the changes directly using GitHub web.

[EchoEllet]

Thank you for your contribution.