String Lexing Fix (Do not lex '\"' as end of the string)

[daeisbae]

Previously when lexing the string "Hello \" World", it will return

>>> "Hello \" World"
Token( TokenType: String Value: 'Hello \" World' )
Token( TokenType: Whitespace Value: ' ' )
Token( TokenType: Identifier Value: 'World' )
Token( TokenType: String Value: '' )
Token( TokenType: End of line Value: '' )

Which is not a expected result. The char '\"' should be converted to double quote inside the string NOT being lexed individually.

Now it is improved to

>>> "Hello \" World"
Token( TokenType: String Value: 'Hello " World' )
Token( TokenType: End of line Value: '' )

Besides the change in string function inside the lexer class, lexing string and null test code is included