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/SyntaxHighlighter.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include <AK/Noncopyable.h>
+#include <AK/WeakPtr.h>
+#include <LibGUI/TextDocument.h>
+
+namespace GUI {
+
+class TextEditor;
+
+class SyntaxHighlighter {
+    AK_MAKE_NONCOPYABLE(SyntaxHighlighter);
+    AK_MAKE_NONMOVABLE(SyntaxHighlighter);
+
+public:
+    virtual ~SyntaxHighlighter();
+
+    virtual void rehighlight() = 0;
+    virtual void highlight_matching_token_pair() = 0;
+
+    void attach(TextEditor& editor);
+    void detach();
+    void cursor_did_change();
+
+protected:
+    SyntaxHighlighter() {}
+
+    WeakPtr<TextEditor> m_editor;
+
+    struct BuddySpan {
+        int index { -1 };
+        GUI::TextDocumentSpan span_backup;
+    };
+
+    bool m_has_brace_buddies { false };
+    BuddySpan m_brace_buddies[2];
+};
+
+}