Add stepwise and time-based script execution with Step and RunFor methods

src/YesNt.Interpreter/Runtime/StepResult.cs

@@ -0,0 +1,27 @@
+namespace YesNt.Interpreter.Runtime;
+
+/// <summary>
+/// The outcome of a single <see cref="YesNtInterpreter.StepOnce"/> call,
+/// or the aggregate result of <see cref="YesNtInterpreter.Step"/> /
+/// <see cref="YesNtInterpreter.RunFor"/>.
+/// </summary>
+public enum StepResult
+{
+    /// <summary>
+    /// A line was executed and more lines remain. Keep stepping to continue.
+    /// </summary>
+    Continue,
+
+    /// <summary>
+    /// The step or time budget was exhausted before the script finished.
+    /// <see cref="YesNtInterpreter.IsRunning"/> is still <see langword="true"/>;
+    /// call any of the run methods again to resume.
+    /// </summary>
+    Paused,
+
+    /// <summary>
+    /// The script ran to completion (end-of-file, explicit exit, or error).
+    /// <see cref="YesNtInterpreter.IsRunning"/> is now <see langword="false"/>.
+    /// </summary>
+    Finished,
+}