[Sequester] Improve command line grammar

[BillWagner]

Now that we've converted to System.CommandLine, we can create a more obvious command line format for Sequester. This would simplify the logic of the app by making the valid combinations of different arguments and options clearer.

See #609

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Associated WorkItem - 593346

[BillWagner]

This should have the following commands:

• Root command: the regular every night run for the past 5 days
  sequester recent --duration 5
• Monthly run for all open issues
  sequester open
• Troubleshoot run for 1 issue
  sequester issue 12345

[BillWagner]

Moving back into the backlog. I'll pick this up when we have a reason to update Sequester for other reasons.

[BillWagner]

Thinking about doing this in July.