support parsing "\\"

In current parser, we cannot parse "\\"
This request fix StringLiteralToken to pass following script.

```
$ cat Test.green
String f() {
    return "\\";
}
$ java -jar GreenTeaScript.jar Test.green
(error) (Test.green:2) expected " to close the string literal
(info) (Test.green:2) stopping script eval at (error) (Test.green:2) expected " to close the string literal
abort loading: Test.green
```