[FR] Bool data type?

Would it be tricky/breaking to add support for a bool/boolean data type? All of the conditional/comparison operators require a boolean value, so users currently have to re-evaluate the condition at each call or rely on the implicit conversion from an int type.

It could make for cleaner syntax to be able to do something like:

data {
  real y_mean;
  bool use_log;
}
parameters {
  real y;
}
model {
  if (use_log) {
    y ~ lognormal(y_mean, 1);
  } else {
    y ~ normal(y_mean, 1);
  }
}

Start by reviewing how conditional and comparison operators, implicit int conversion, and declarations are represented in the Stan transpiler. Define the required syntax and type-checking behavior, then verify that the example compiles and behaves correctly with regression coverage.