feat(base_note): ✨ add next and previous getters (#445)

lib/src/note/base_note.dart

@@ -141,11 +141,31 @@ enum BaseNote implements Comparable<BaseNote> {
   /// ```dart
   /// BaseNote.g.transposeBySize(Size.unison) == BaseNote.g
   /// BaseNote.g.transposeBySize(Size.fifth) == BaseNote.d
-  /// BaseNote.a.transposeBySize(-3) == BaseNote.f
+  /// BaseNote.a.transposeBySize(-Size.third) == BaseNote.f
   /// ```
   BaseNote transposeBySize(Size size) =>
       BaseNote.fromOrdinal(ordinal + size.incrementBy(-1));
 
+  /// The next ordinal [BaseNote].
+  ///
+  /// Example:
+  /// ```dart
+  /// BaseNote.c.next == BaseNote.d
+  /// BaseNote.f.next == BaseNote.a
+  /// BaseNote.b.next == BaseNote.c
+  /// ```
+  BaseNote get next => transposeBySize(Size.second);
+
+  /// The previous ordinal [BaseNote].
+  ///
+  /// Example:
+  /// ```dart
+  /// BaseNote.e.previous == BaseNote.d
+  /// BaseNote.g.previous == BaseNote.f
+  /// BaseNote.c.previous == BaseNote.b
+  /// ```
+  BaseNote get previous => transposeBySize(-Size.second);
+
   /// The string representation of this [BaseNote] based on [system].
   ///
   /// See [NoteNotation] for all system implementations.

test/src/note/base_note_test.dart

@@ -95,6 +95,24 @@ void main() {
       });
     });
 
+    group('.next', () {
+      test('returns the next ordinal BaseNote', () {
+        expect(BaseNote.c.next, BaseNote.d);
+        expect(BaseNote.e.next, BaseNote.f);
+        expect(BaseNote.f.next, BaseNote.g);
+        expect(BaseNote.b.next, BaseNote.c);
+      });
+    });
+
+    group('.previous', () {
+      test('returns the previous ordinal BaseNote', () {
+        expect(BaseNote.b.previous, BaseNote.a);
+        expect(BaseNote.f.previous, BaseNote.e);
+        expect(BaseNote.d.previous, BaseNote.c);
+        expect(BaseNote.c.previous, BaseNote.b);
+      });
+    });
+
     group('.compareTo()', () {
       test('sorts BaseNotes in a collection', () {
         final orderedSet = SplayTreeSet<BaseNote>.of({