Moved the build script to System.CommandLine (#1159)

app/Build/Tools/CommandExitedException.cs

@@ -0,0 +1,19 @@
+namespace Build.Tools;
+
+/// <summary>
+/// Ends the build script with an exit code, after the command has said what went wrong.
+/// </summary>
+/// <remarks>
+/// A release runs several commands one after another, and a failure deep inside one of them has to end
+/// the whole release, so that nothing is built on top of half-prepared metadata or broken artifacts.
+/// Program.cs turns the exception into the exit code and prints nothing more: the command already
+/// explained the failure, and a stack trace would only bury that explanation.
+/// </remarks>
+/// <param name="exitCode">The exit code of the build script, where anything but zero is a failure.</param>
+public sealed class CommandExitedException(int exitCode) : Exception
+{
+    /// <summary>
+    /// The exit code the build script ends with.
+    /// </summary>
+    public int ExitCode { get; } = exitCode;
+}