Optional
stringCheckFn: (value: string) => value is TIf given and the supplied value is a string other than a boolean string, then it is called with the value as a single argument. It must return true if the value is valid and false otherwise. If it is not given, then any literal string is accepted.
Returns a valid boolean or a string from the supplied value, ensuring the supplied value is either a boolean or boolean string (see validateBoolean), or a string that conforms to the given
checkFn
.Throws
LisnUsageError If the value is invalid.