remove the legacy double dash behavior and `ParseResult.UnparsedTokens`

[jonsequitur]

This functionality exists primarily to support the pre-existing behavior of the dotnet CLI, but that can likely be reimplemented using Argument<string[]>.

[baronfel]

As a case study for users of this library, here is a commit where explicit -- behavior was superseded with an Argument<string[]>.

[baronfel]

The SDK is working on removing all use of the legacy -- handling for .NET 7, so it's just a matter of timing.

[Tunduk]

@jonsequitur Hi!
dotnet/sdk team merged PR with removing legacy double dash behavior.
If you don't mind I want to try to fix this issue.

[jonsequitur]

Closed by #1806.