Function boa::builtins::string::index_of

pub fn index_of(
    this: &Value, 
    args: &[Value], 
    ctx: &mut Interpreter
) -> ResultValue

If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, then the smallest such index is returned; otherwise, -1 is returned. If position is undefined, 0 is assumed, so as to search all of the String. https://tc39.github.io/ecma262/#sec-string.prototype.includes