refactor!(quality): ♻️ rename abbreviation → symbol getter (#356)

* refactor!(quality): ♻️ rename `abbreviation` → `symbol` getter

* docs(quality): 📖 use `Quality` identifier for commented `_augmentedSymbol`

lib/src/interval/interval.dart

@@ -444,11 +444,11 @@ final class Interval implements Comparable<Interval> {
 
   @override
   String toString() {
-    final naming = '${quality.abbreviation}${size.abs()}';
+    final naming = '${quality.symbol}${size.abs()}';
     final descendingAbbreviation = isDescending ? 'desc ' : '';
     if (isCompound) {
       return '$descendingAbbreviation$naming '
-          '(${quality.abbreviation}${simplified.size.abs()})';
+          '(${quality.symbol}${simplified.size.abs()})';
     }
 
     return '$descendingAbbreviation$naming';

lib/src/interval/quality.dart

@@ -19,14 +19,14 @@ sealed class Quality implements Comparable<Quality> {
   static const _diminishedSymbol = 'd';
   static const _augmentedSymbol = 'A';
 
-  /// The textual abbreviation of this [Quality].
-  String get abbreviation;
+  /// The symbol of this [Quality].
+  String get symbol;
 
   /// Returns the inverted version of this [Quality].
   Quality get inverted;
 
   @override
-  String toString() => '$abbreviation (${semitones.toDeltaString()})';
+  String toString() => '$symbol (${semitones.toDeltaString()})';
 
   @override
   bool operator ==(Object other) =>
@@ -100,12 +100,12 @@ class PerfectQuality extends Quality {
     return switch (source[0]) {
       Quality._diminishedSymbol => PerfectQuality(-source.length),
       _perfectSymbol => PerfectQuality.perfect,
-      _ /* _augmentedSymbol */ => PerfectQuality(source.length),
+      _ /* Quality._augmentedSymbol */ => PerfectQuality(source.length),
     };
   }
 
   @override
-  String get abbreviation => switch (semitones) {
+  String get symbol => switch (semitones) {
         < 0 => Quality._diminishedSymbol * semitones.abs(),
         0 => _perfectSymbol,
         _ => Quality._augmentedSymbol * semitones,
@@ -189,12 +189,12 @@ class ImperfectQuality extends Quality {
       Quality._diminishedSymbol => ImperfectQuality(-source.length),
       _minorSymbol => ImperfectQuality.minor,
       _majorSymbol => ImperfectQuality.major,
-      _ /* _augmentedSymbol */ => ImperfectQuality(source.length + 1),
+      _ /* Quality._augmentedSymbol */ => ImperfectQuality(source.length + 1),
     };
   }
 
   @override
-  String get abbreviation => switch (semitones) {
+  String get symbol => switch (semitones) {
         < 0 => Quality._diminishedSymbol * semitones.abs(),
         0 => _minorSymbol,
         1 => _majorSymbol,