LibSQL: Move Lexer and Parser machinery to AST directory

The SQL engine is expected to be a fairly sizeable piece of software.
Therefore we're starting to restructure the codebase for growth.

Userland/Libraries/LibSQL/AST/Lexer.h

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "Token.h"
+#include <AK/HashMap.h>
+#include <AK/String.h>
+#include <AK/StringView.h>
+
+namespace SQL::AST {
+
+class Lexer {
+public:
+    explicit Lexer(StringView source);
+
+    Token next();
+
+private:
+    void consume();
+
+    bool consume_whitespace_and_comments();
+    bool consume_numeric_literal();
+    bool consume_string_literal();
+    bool consume_blob_literal();
+    bool consume_exponent();
+    bool consume_hexadecimal_number();
+
+    bool match(char a, char b) const;
+    bool is_identifier_start() const;
+    bool is_identifier_middle() const;
+    bool is_numeric_literal_start() const;
+    bool is_string_literal_start() const;
+    bool is_string_literal_end() const;
+    bool is_blob_literal_start() const;
+    bool is_line_comment_start() const;
+    bool is_block_comment_start() const;
+    bool is_block_comment_end() const;
+    bool is_line_break() const;
+    bool is_eof() const;
+
+    static HashMap<String, TokenType> s_keywords;
+    static HashMap<char, TokenType> s_one_char_tokens;
+    static HashMap<String, TokenType> s_two_char_tokens;
+
+    StringView m_source;
+    size_t m_line_number { 1 };
+    size_t m_line_column { 0 };
+    char m_current_char { 0 };
+    bool m_eof { false };
+    size_t m_position { 0 };
+};
+
+}