Split Settings into model, serializer, paths and store

698 lines did all of it at once. The mutable CaptureFromUi callback becomes an
ISettingsCapture the settings view implements.

StarterNG/Infrastructure/Adapters/SystemEnvironment.cs

@@ -0,0 +1,18 @@
+using System;
+using StarterNG.Application.Abstractions;
+
+namespace StarterNG.Infrastructure.Adapters;
+
+/// <summary><see cref="IEnvironment"/> backed by the running process.</summary>
+public sealed class SystemEnvironment : IEnvironment
+{
+    public bool IsWindows => OperatingSystem.IsWindows();
+
+    public bool IsLinux => OperatingSystem.IsLinux();
+
+    public bool IsMacOS => OperatingSystem.IsMacOS();
+
+    public string? GetVariable(string name) => Environment.GetEnvironmentVariable(name);
+
+    public string BaseDirectory => AppContext.BaseDirectory;
+}