LibGUI+HackStudio: Move syntax highlighting from HackStudio to LibGUI

This patch introduces the GUI::SyntaxHighlighter class, which can be
attached to a GUI::TextEditor to provide syntax highlighting.

The C++ syntax highlighting from HackStudio becomes a new class called
GUI::CppSyntaxHighlighter. This will make it possible to get C++ syntax
highlighting in any app that uses a GUI::TextEditor. :^)

Sidenote: It does feel a bit weird having a C++ lexer in a GUI toolkit
library, and we'll probably end up moving this out to a separate place
as this functionality grows larger.

Libraries/LibGUI/CppSyntaxHighlighter.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include <LibGUI/SyntaxHighlighter.h>
+
+namespace GUI {
+
+class CppSyntaxHighlighter final : public SyntaxHighlighter {
+public:
+    CppSyntaxHighlighter() {}
+
+    virtual ~CppSyntaxHighlighter() override;
+    virtual void rehighlight() override;
+    virtual void highlight_matching_token_pair() override;
+};
+
+}