Shell: Move line editing to a separate class.

To be clear, there isn't really any line editing yet. But there is
going to be, so let's have it in its own class.

Shell/LineEditor.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <AK/AKString.h>
+#include <AK/Vector.h>
+
+class LineEditor {
+public:
+    LineEditor();
+    ~LineEditor();
+
+    String get_line();
+
+    void add_to_history(const String&);
+    const Vector<String>& history() const { return m_history; }
+
+private:
+    Vector<char, 1024> m_buffer;
+    int m_cursor { 0 };
+
+    // FIXME: This should be something more take_first()-friendly.
+    Vector<String> m_history;
+    int m_history_capacity { 100 };
+};