Replace lexertl with a custom lexeter implementation

QueryLexeter does not parse "atWord" because I couldn't find what it is used for.

YACReaderLibrary/db/query_lexer.h

@@ -0,0 +1,59 @@
+#ifndef QUERY_LEXER_H
+#define QUERY_LEXER_H
+
+#include <string>
+
+class Token
+{
+public:
+    enum class Type {
+        eof,
+        opcode,
+        atWord,
+        word,
+        quotedWord,
+        space
+    };
+
+    Token(Type type, std::string lexeme = "")
+        : _type(type), _lexeme(std::move(lexeme))
+    {
+    }
+
+    Type type() const
+    {
+        return _type;
+    }
+
+    std::string lexeme() const
+    {
+        return _lexeme;
+    }
+
+private:
+    Type _type {};
+    std::string _lexeme {};
+};
+
+class QueryLexer
+{
+public:
+    QueryLexer(const std::string &input);
+    Token next();
+
+private:
+    std::string input;
+    int index = 0;
+
+    char peek();
+    char get();
+
+    Token single(Token::Type type);
+    Token space();
+    Token word();
+    Token quotedWord();
+
+    bool isSpace(char c);
+};
+
+#endif // QUERY_LEXER_H