LibLine: Add live styling support

This patchset adds an stylization interface to LibLine, and breaks
multiline editing.
With the most adorable Style constructor I've ever seen :^)

Libraries/LibLine/Style.h

@@ -0,0 +1,108 @@
+/*
+ * Copyright (c) 2020, The SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+#include <stdlib.h>
+
+namespace Line {
+
+class Style {
+public:
+    enum class Color : int {
+        Default = 9,
+        Black = 0,
+        Red,
+        Green,
+        Yellow,
+        Blue,
+        Magenta,
+        Cyan,
+        White,
+        // TODO: it appears that we do not support these SGR options
+        BrightBlack = 60,
+        BrightRed,
+        BrightGreen,
+        BrightYellow,
+        BrightBlue,
+        BrightMagenta,
+        BrightCyan,
+        BrightWhite,
+    };
+
+    struct UnderlineTag {
+    };
+    struct BoldTag {
+    };
+    struct ItalicTag {
+    };
+    struct Background {
+        explicit Background(Color color)
+            : m_color(color)
+        {
+        }
+        Color m_color;
+    };
+    struct Foreground {
+        explicit Foreground(Color color)
+            : m_color(color)
+        {
+        }
+        Color m_color;
+    };
+
+    static constexpr UnderlineTag Underline {};
+    static constexpr BoldTag Bold {};
+    static constexpr ItalicTag Italic {};
+
+    // prepare for the horror of templates
+    template <typename T, typename... Rest>
+    Style(const T& style_arg, Rest... rest)
+        : Style(rest...)
+    {
+        set(style_arg);
+    }
+    Style() {}
+
+    bool underline() const { return m_underline; }
+    bool bold() const { return m_bold; }
+    bool italic() const { return m_italic; }
+    Color background() const { return m_background; }
+    Color foreground() const { return m_foreground; }
+
+    void set(const ItalicTag&) { m_italic = true; }
+    void set(const BoldTag&) { m_bold = true; }
+    void set(const UnderlineTag&) { m_underline = true; }
+    void set(const Background& bg) { m_background = bg.m_color; }
+    void set(const Foreground& fg) { m_foreground = fg.m_color; }
+
+private:
+    bool m_underline { false };
+    bool m_bold { false };
+    bool m_italic { false };
+    Color m_background { Color::Default };
+    Color m_foreground { Color::Default };
+};
+}