fix: error while compiling y.tab.c file

[TanKaiYi0220]

This change resolves #1 and issues of missing or conflict declarations by following steps:

1. removing the #include"lex.yy.c" from yacc file. (For Part 2-6)
2. adding declarations of char * yytext; before using in yacc file. (For Part 2-6)
3. Adjust the order of declarations of variables and functions. (For Part 3-6)

Also, updated README.md by adding the information of environment which is used.

Thanks for your explanation for each step.