Decouple drivers from terminal sequence construction.

Also, refactor some core types to make unit testing more practical, e.g. you
can now create a custom VirtualTerminal.

Fixes #22.
Fixes #19.
Fixes #32.

src/core/Drivers/TerminalDriver.cs

@@ -51,8 +51,6 @@ public TerminalSize Size
 
     readonly object _rawLock = new();
 
-    readonly object _sequenceLock = new();
-
     readonly ManualResetEventSlim _event = new();
 
     [SuppressMessage("Style", "IDE0052")]

src/core/SystemVirtualTerminal.cs

@@ -0,0 +1,57 @@
+using System.Drivers;
+using System.Drivers.Unix;
+using System.Drivers.Windows;
+
+namespace System;
+
+public sealed class SystemVirtualTerminal : VirtualTerminal
+{
+    public override event Action<TerminalSize>? Resize
+    {
+        add => _driver.Resize += value;
+        remove => _driver.Resize -= value;
+    }
+
+    public override event Action<TerminalSignalContext>? Signal
+    {
+        add => _driver.Signal += value;
+        remove => _driver.Signal -= value;
+    }
+
+    public static SystemVirtualTerminal Instance { get; } = new();
+
+    public override TerminalReader StdIn => _driver.StdIn;
+
+    public override TerminalWriter StdOut => _driver.StdOut;
+
+    public override TerminalWriter StdError => _driver.StdError;
+
+    public override bool IsRawMode => _driver.IsRawMode;
+
+    public override TerminalSize Size => _driver.Size;
+
+    readonly TerminalDriver _driver =
+        OperatingSystem.IsLinux() ? LinuxTerminalDriver.Instance :
+        OperatingSystem.IsMacOS() ? MacOSTerminalDriver.Instance :
+        OperatingSystem.IsWindows() ? WindowsTerminalDriver.Instance :
+        throw new TerminalException("This platform is not supported.");
+
+    SystemVirtualTerminal()
+    {
+    }
+
+    public override void GenerateSignal(TerminalSignal signal)
+    {
+        _driver.GenerateSignal(signal);
+    }
+
+    public override void EnableRawMode()
+    {
+        _driver.EnableRawMode();
+    }
+
+    public override void DisableRawMode()
+    {
+        _driver.DisableRawMode();
+    }
+}