Update bashly schema to support `flag.unique` and `arg.unique`

[DannyBen]

See #454 and #456.

Both Argument and Flag now support a new boolean option named unique.
The JSON schema needs to be updated accordingly.

[DannyBen]

@EmilyGraceSeville7cf - do you have time for this?

[EmilyGraceSeville7cf]

Hey. Yeah, I guess I can do it tomorrow.🙂 Sorry for the long response.