Validates whether the input can be used as a string.
v::stringVal()->assert('6');
// Validation passes successfully
v::stringVal()->assert('String');
// Validation passes successfully
v::stringVal()->assert(1.0);
// Validation passes successfully
v::stringVal()->assert(42);
// Validation passes successfully
v::stringVal()->assert(false);
// Validation passes successfully
v::stringVal()->assert(true);
// Validation passes successfully
v::stringVal()->assert(new ClassWithToString());
// Validation passes successfully