Store AnnotationsId prefix as separate field

Avoiding the concat will marginally reduce the amount of heap memory,
but more importantly makes it cleaner for annotation providers to
check the prefix of the requested annotation ID.

Author: mhutch

src/System.CommandLine.Subsystems/Subsystems/AnnotationId.cs

@@ -6,4 +6,7 @@ namespace System.CommandLine.Subsystems;
 /// <summary>
 /// Describes the ID and type of an annotation.
 /// </summary>
-public record struct AnnotationId<TValue>(string Id);
+public record struct AnnotationId<TValue>(string Prefix, string Id)
+{
+    public override readonly string ToString() => $"{Prefix}.{Id}";
+}

src/System.CommandLine.Subsystems/Subsystems/Annotations/HelpAnnotations.cs

@@ -8,7 +8,7 @@ namespace System.CommandLine.Subsystems.Annotations;
 /// </summary>
 public static class HelpAnnotations
 {
-    // I made this public because we need an identifier for subsystem Kind, and I think this makes a very good one.
-    public static string Prefix { get; } = "Help.";
-    public static AnnotationId<string> Description { get; } = new(Prefix + nameof(Description));
+    public static string Prefix { get; } = nameof(SubsystemKind.Help);
+
+    public static AnnotationId<string> Description { get; } = new(Prefix, nameof(Description));
 }

src/System.CommandLine.Subsystems/Subsystems/Annotations/VersionAnnotations.cs

@@ -8,7 +8,7 @@ namespace System.CommandLine.Subsystems.Annotations;
 /// </summary>
 public static class VersionAnnotations
 {
-    // I made this public because we need an identifier for subsystem Kind, and I think this makes a very good one.
-    public static string Prefix { get; } = "Version."; // TODO: Decide between const and property. Isn't a const baked in at the callsite? Would that be OK when this is public?
-    public static AnnotationId<string> Version { get; } = new(Prefix + nameof(Version));
+    public static string Prefix { get; } = nameof(SubsystemKind.Version);
+
+    public static AnnotationId<string> Version { get; } = new(Prefix, nameof(Version));
 }