Make the CLI parser not barf on unknown tokens

[baronfel]

Per #952 unknown tokens cause exceptions. We should be more flexible and ignore them instead of hard erroring to make backwards-compatibility easier.

[TheAngryByrd]

I think https://learn.microsoft.com/en-us/dotnet/api/system.commandline.command.treatunmatchedtokensaserrors?view=system-commandline might be what we need to set. Otherwise we probably want to keep old flags around and create issues for clients when we deprecate them.