Add double harmonic major scale pattern

With documentation, Wikipedia diagram, and unit tests for the scale notes, name and toString()

lib/src/scale/scale_pattern.dart

@@ -231,6 +231,19 @@ final class ScalePattern {
     Interval.m2,
   ]);
 
+  /// See [Double harmonic scale](https://en.wikipedia.org/wiki/Double_harmonic_scale).
+  ///
+  /// ![C Double harmonic scale](https://upload.wikimedia.org/score/r/f/rf4xfu7bhu0k9n0ccidte6yjngjswnm/rf4xfu7b.png).
+  static const doubleHarmonicMajor = ScalePattern([
+    Interval.m2,
+    Interval.A2,
+    Interval.m2,
+    Interval.M2,
+    Interval.m2,
+    Interval.A2,
+    Interval.m2,
+  ]);
+
   /// Creates a new [ScalePattern] from the given [chordPattern].
   ///
   /// Example:
@@ -447,6 +460,7 @@ final class ScalePattern {
         majorPentatonic => 'Major pentatonic',
         minorPentatonic => 'Minor pentatonic',
         octatonic => 'Octatonic',
+        doubleHarmonicMajor => 'Double harmonic major',
         _ => null,
       };
 

test/src/scale/scale_pattern_test.dart

@@ -382,6 +382,35 @@ void main() {
           Scale([Note.g, Note.b.flat, Note.c, Note.d, Note.f, Note.g]),
         );
       });
+
+      test('returns the double harmonic major Scale on Note', () {
+        expect(
+          ScalePattern.doubleHarmonicMajor.on(Note.c),
+          Scale([
+            Note.c,
+            Note.d.flat,
+            Note.e,
+            Note.f,
+            Note.g,
+            Note.a.flat,
+            Note.b,
+            Note.c,
+          ]),
+        );
+        expect(
+          ScalePattern.doubleHarmonicMajor.on(Note.f.sharp),
+          Scale([
+            Note.f.sharp,
+            Note.g,
+            Note.a.sharp,
+            Note.b,
+            Note.c.sharp,
+            Note.d,
+            Note.e.sharp,
+            Note.f.sharp,
+          ]),
+        );
+      });
     });
 
     group('.mirrored', () {
@@ -573,6 +602,7 @@ void main() {
         expect(ScalePattern.majorPentatonic.name, 'Major pentatonic');
         expect(ScalePattern.minorPentatonic.name, 'Minor pentatonic');
         expect(ScalePattern.octatonic.name, 'Octatonic');
+        expect(ScalePattern.doubleHarmonicMajor.name, 'Double harmonic major');
       });
     });
 
@@ -606,6 +636,10 @@ void main() {
           ScalePattern.octatonic.toString(),
           'Octatonic (M2 m2 M2 m2 M2 m2 M2 m2)',
         );
+        expect(
+          ScalePattern.doubleHarmonicMajor.toString(),
+          'Double harmonic major (m2 A2 m2 M2 m2 A2 m2)',
+        );
       });
     });