refactor(interval): ♻️ rewrite Size as extension type

README.md

@@ -67,8 +67,8 @@ Pitch.parse('Eb3'); // E♭3
 Create an `Interval`:
 
 ```dart
-const Interval.perfect(5, PerfectQuality.perfect); // P5
 Interval.P4; // P4
+const Interval.perfect(Size.twelfth, PerfectQuality.perfect); // P12
 ```
 
 Or turn it descending:

example/main.dart

@@ -21,8 +21,8 @@ void main() {
   Pitch.parse('Eb3'); // E♭3
 
   // Intervals
-  const Interval.perfect(15, PerfectQuality.perfect); // P15
   Interval.P4; // P4
+  const Interval.perfect(Size.twelfth, PerfectQuality.perfect); // P12
 
   -Interval.m6; // desc m6
   Interval.M3.descending(); // desc M3

lib/music_notes.dart

@@ -9,6 +9,7 @@ export 'src/harmony/harmonic_function.dart';
 export 'src/interval/interval.dart';
 export 'src/interval/interval_class.dart';
 export 'src/interval/quality.dart';
+export 'src/interval/size.dart';
 export 'src/key/key.dart';
 export 'src/key/key_signature.dart';
 export 'src/key/mode.dart';

lib/src/chordable.dart

@@ -1,6 +1,7 @@
 import 'harmony/chord.dart';
 import 'interval/interval.dart';
 import 'interval/quality.dart';
+import 'interval/size.dart';
 
 /// A mixin for items that can form chords.
 ///
@@ -28,23 +29,23 @@ mixin Chordable<T> {
 
   /// Returns a new [T] adding a [quality] 6th.
   T add6([ImperfectQuality quality = ImperfectQuality.major]) =>
-      add(Interval.imperfect(6, quality));
+      add(Interval.imperfect(Size.sixth, quality));
 
   /// Returns a new [T] adding a [quality] 7th.
   T add7([ImperfectQuality quality = ImperfectQuality.minor]) =>
-      add(Interval.imperfect(7, quality));
+      add(Interval.imperfect(Size.seventh, quality));
 
   /// Returns a new [T] adding a [quality] 9th.
   T add9([ImperfectQuality quality = ImperfectQuality.major]) =>
-      add(Interval.imperfect(9, quality));
+      add(Interval.imperfect(Size.ninth, quality));
 
   /// Returns a new [T] adding an [quality] 11th.
   T add11([PerfectQuality quality = PerfectQuality.perfect]) =>
-      add(Interval.perfect(11, quality));
+      add(Interval.perfect(Size.eleventh, quality));
 
   /// Returns a new [T] adding a [quality] 13th.
   T add13([ImperfectQuality quality = ImperfectQuality.major]) =>
-      add(Interval.imperfect(13, quality));
+      add(Interval.imperfect(Size.thirteenth, quality));
 
   /// Returns a new [T] adding [interval].
   T add(Interval interval, {Set<int>? replaceSizes});