Start working on virtual consoles/TTYs.

This is a mess right now, but I'd rather commit as I go.

Kernel/VirtualConsole.h

@@ -0,0 +1,61 @@
+#pragma once
+
+#include "TTY.h"
+#include "Keyboard.h"
+#include "Console.h"
+
+class VirtualConsole final : public TTY, public KeyboardClient, public ConsoleImplementation {
+public:
+    enum InitialContents { Cleared, AdoptCurrentVGABuffer };
+
+    VirtualConsole(unsigned index, InitialContents = Cleared);
+    virtual ~VirtualConsole() override;
+
+    void adoptCurrentVGABuffer();
+    void setActive(bool);
+
+    static void switchTo(unsigned);
+    static void initialize();
+
+private:
+    // ^KeyboardClient
+    void onKeyPress(byte) override;
+
+    // ^ConsoleImplementation
+    void onConsoleReceive(byte) override;
+
+    void onChar(byte);
+
+    byte* m_buffer;
+    unsigned m_index;
+    bool m_active { false };
+
+    void escape$H(const Vector<unsigned>&);
+    void escape$J(const Vector<unsigned>&);
+    void escape$m(const Vector<unsigned>&);
+    void escape$s(const Vector<unsigned>&);
+    void escape$u(const Vector<unsigned>&);
+
+    const byte m_rows { 25 };
+    const byte m_columns { 80 };
+    byte m_cursorRow { 0 };
+    byte m_cursorColumn { 0 };
+    byte m_savedCursorRow { 0 };
+    byte m_savedCursorColumn { 0 };
+    byte m_currentAttribute { 0x07 };
+
+    void executeEscapeSequence(byte final);
+
+    enum EscapeState {
+        Normal,
+        ExpectBracket,
+        ExpectParameter,
+        ExpectIntermediate,
+        ExpectFinal,
+    };
+    EscapeState m_escState { Normal };
+    Vector<byte> m_parameters;
+    Vector<byte> m_intermediates;
+
+};
+