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/Application/Abstractions/IEnvironment.cs

@@ -0,0 +1,20 @@
+namespace StarterNG.Application.Abstractions;
+
+/// <summary>
+/// The bits of the host machine the starter has to ask about: which operating
+/// system it is on, and where that system keeps per-user configuration.
+/// </summary>
+public interface IEnvironment
+{
+    bool IsWindows { get; }
+
+    bool IsLinux { get; }
+
+    bool IsMacOS { get; }
+
+    /// <summary>An environment variable, or null when it is not set.</summary>
+    string? GetVariable(string name);
+
+    /// <summary>The folder the starter's own executable lives in.</summary>
+    string BaseDirectory { get; }
+}

StarterNG/Application/Abstractions/IFileSystem.cs

@@ -58,4 +58,10 @@ public interface IFileSystem
     IReadOnlyList<string> GetFilesRecursive(string path, string searchPattern);
 
     DateTime GetLastWriteTimeUtc(string path);
+
+    /// <summary>
+    /// Whether the file carries an execute bit. Always true on Windows, where
+    /// executability is a property of the file's contents rather than its mode.
+    /// </summary>
+    bool IsExecutable(string path);
 }