refactor: move LineHeightAttribute class to custom_attributes.dart

lib/src/document/attribute.dart

@@ -4,6 +4,9 @@ import 'package:equatable/equatable.dart';
 import 'package:meta/meta.dart' show immutable;
 import 'package:quiver/core.dart';
 
+import 'custom_attributes.dart';
+export 'custom_attributes.dart';
+
 enum AttributeScope {
   inline, // refer to https://quilljs.com/docs/formats/#inline
   block, // refer to https://quilljs.com/docs/formats/#block
@@ -354,27 +357,6 @@ class HeaderAttribute extends Attribute<int?> {
       : super('header', AttributeScope.block, level);
 }
 
-/// This attribute represents the space between text lines. The line height can be
-/// adjusted using predefined constants or custom values
-///
-/// The attribute at the json looks like: "attributes":{"line-height": 1.5 }
-class LineHeightAttribute extends Attribute<double?> {
-  const LineHeightAttribute({double? lineHeight})
-      : super('line-height', AttributeScope.block, lineHeight);
-
-  static const Attribute<double?> lineHeightNormal =
-      LineHeightAttribute(lineHeight: 1);
-
-  static const Attribute<double?> lineHeightTight =
-      LineHeightAttribute(lineHeight: 1.15);
-
-  static const Attribute<double?> lineHeightOneAndHalf =
-      LineHeightAttribute(lineHeight: 1.5);
-
-  static const Attribute<double?> lineHeightDouble =
-      LineHeightAttribute(lineHeight: 2);
-}
-
 class IndentAttribute extends Attribute<int?> {
   const IndentAttribute({int? level})
       : super('indent', AttributeScope.block, level);

lib/src/document/custom_attributes.dart

@@ -0,0 +1,25 @@
+import 'attribute.dart';
+
+// Attributes that don't conform to standard Quill Delta
+// and are not compatible with https://quilljs.com/docs/delta/
+
+/// This attribute represents the space between text lines. The line height can be
+/// adjusted using predefined constants or custom values
+///
+/// The attribute at the json looks like: "attributes":{"line-height": 1.5 }
+class LineHeightAttribute extends Attribute<double?> {
+  const LineHeightAttribute({double? lineHeight})
+      : super('line-height', AttributeScope.block, lineHeight);
+
+  static const Attribute<double?> lineHeightNormal =
+      LineHeightAttribute(lineHeight: 1);
+
+  static const Attribute<double?> lineHeightTight =
+      LineHeightAttribute(lineHeight: 1.15);
+
+  static const Attribute<double?> lineHeightOneAndHalf =
+      LineHeightAttribute(lineHeight: 1.5);
+
+  static const Attribute<double?> lineHeightDouble =
+      LineHeightAttribute(lineHeight: 2);
+}