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.cpp

@@ -0,0 +1,82 @@
+/*
+ * Copyright (c) 2022, Brian Gianforcaro <bgianf@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/CharacterTypes.h>
+#include <AK/Vector.h>
+#include <LibGUI/GitCommitLexer.h>
+
+namespace GUI {
+
+GitCommitLexer::GitCommitLexer(StringView input)
+    : m_input(input)
+{
+}
+
+char GitCommitLexer::peek(size_t offset) const
+{
+    if ((m_index + offset) >= m_input.length())
+        return 0;
+    return m_input[m_index + offset];
+}
+
+char GitCommitLexer::consume()
+{
+    VERIFY(m_index < m_input.length());
+    char ch = m_input[m_index++];
+    if (ch == '\n') {
+        m_position.line++;
+        m_position.column = 0;
+    } else {
+        m_position.column++;
+    }
+    return ch;
+}
+
+Vector<GitCommitToken> GitCommitLexer::lex()
+{
+    Vector<GitCommitToken> tokens;
+
+    size_t token_start_index = 0;
+    GitCommitPosition token_start_position;
+
+    auto begin_token = [&] {
+        token_start_index = m_index;
+        token_start_position = m_position;
+    };
+
+    auto commit_token = [&](auto type) {
+        GitCommitToken token;
+        token.m_view = m_input.substring_view(token_start_index, m_index - token_start_index);
+        token.m_type = type;
+        token.m_start = token_start_position;
+        token.m_end = m_position;
+        tokens.append(token);
+    };
+
+    while (m_index < m_input.length()) {
+        if (is_ascii_space(peek(0))) {
+            begin_token();
+            while (is_ascii_space(peek()))
+                consume();
+            continue;
+        }
+
+        // Commit comments
+        if (peek(0) && peek(0) == '#') {
+            begin_token();
+            while (peek() && peek() != '\n')
+                consume();
+            commit_token(GitCommitToken::Type::Comment);
+            continue;
+        }
+
+        consume();
+        commit_token(GitCommitToken::Type::Unknown);
+    }
+    return tokens;
+}
+
+}