LibCMake: Add a lexer for CMakeCache.txt

This is a totally different syntax than for regular CMake files, and
also is undocumented and subject to change, but it's also nice and
simple. :^)

Userland/Libraries/LibCMake/CMakeCache/Lexer.h

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2023, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/GenericLexer.h>
+#include <AK/Vector.h>
+#include <LibCMake/CMakeCache/Token.h>
+
+namespace CMake::Cache {
+
+class Lexer : private GenericLexer {
+public:
+    static ErrorOr<Vector<Token>> lex(StringView input);
+
+private:
+    Lexer(StringView input);
+
+    ErrorOr<Vector<Token>> lex_file();
+
+    void skip_whitespace();
+
+    void consume_comment();
+    void consume_help_text();
+    void consume_variable_definition();
+    void consume_key();
+    void consume_colon();
+    void consume_type();
+    void consume_equals();
+    void consume_value();
+    void consume_garbage();
+
+    Position position() const;
+    void next_line();
+
+    void emit_token(Token::Type, StringView value, Position start, Position end);
+
+    Vector<Token> m_tokens;
+    size_t m_line { 0 };
+    size_t m_string_offset_after_previous_newline { 0 };
+};
+
+}