This adds the special SEXP return values (SEXP_KNOWN_TRUE, SEXP_NAN, and so forth) as enums, and adds code to intercept these enums if they are returned where the code would normally expect a boolean or number. If an enum is found, it is converted to its special return value equivalent.
With this change, a script can end a function with `return SEXP_KNOWN_TRUE` or similar. No other API changes are needed; in particular, no special `$Return Type:`s are needed.
Implements #5959.
Goober5000
897e388d1c