Skip String Literals in OperationsChecker

[rcosta358]

This PR is similar to #63.
There was a problem where this simple program failed parsing:

public class Example {
    void example() {
        System.out.println("Hello, " + "World!");
    }
}

Syntax error with message
Found 1 error, with the message:
* Error in no viable alternative at input 'Hello,', in the position 5

This was because strings were not being handled in the binary operations, in this case string concatenation.
It was fixed by skipping string literals in the OperationsChecker.

[alcides]

FYI, o SMT tem um suporte limitado para string e concatenação de strings (e tamanho de strings).

Tal como 1 é selfified para {x:Int | x == 1}, "ola" devia ser selfified para {x:String | x == "ola"}.

Imagino que o Z3 Java tenha um binding correspondente.

[CatarinaGamboa]

Good catch! What a strange error 😅