boolean type is not inferred from default

When specifying an option:

```ts
command.option("flag", {
  default: false
})
```

Typing works, but yargs type is not inferred.

e.g. when running with `node app.js --flag x`, the `args.flag` has a value of `"x"` instead of throwing an error (as it would with `"type": "boolean"`)