LibGUI: Add a GitCommit SyntaxHighlighter implementation

This highlighter just syntax highlights the commented lines in your git
commit message. It could potentially be enhanced to handle the rebase
UI or other more advanced cases in the future.

Userland/Libraries/LibGUI/GitCommitLexer.h

@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) 2022, Brian Gianforcaro <bgianf@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/StringView.h>
+
+namespace GUI {
+
+#define FOR_EACH_TOKEN_TYPE \
+    __TOKEN(Comment)        \
+    __TOKEN(Unknown)
+
+struct GitCommitPosition {
+    size_t line;
+    size_t column;
+};
+
+struct GitCommitToken {
+    enum class Type {
+#define __TOKEN(x) x,
+        FOR_EACH_TOKEN_TYPE
+#undef __TOKEN
+    };
+
+    char const* to_string() const
+    {
+        switch (m_type) {
+#define __TOKEN(x) \
+    case Type::x:  \
+        return #x;
+            FOR_EACH_TOKEN_TYPE
+#undef __TOKEN
+        }
+        VERIFY_NOT_REACHED();
+    }
+
+    Type m_type { Type::Unknown };
+    StringView m_view;
+    GitCommitPosition m_start;
+    GitCommitPosition m_end;
+};
+
+class GitCommitLexer {
+public:
+    GitCommitLexer(StringView);
+
+    Vector<GitCommitToken> lex();
+
+private:
+    char peek(size_t offset = 0) const;
+    char consume();
+
+    StringView m_input;
+    size_t m_index { 0 };
+    GitCommitPosition m_position { 0, 0 };
+};
+
+}