Report on previous dates

[joshuanapoli]

Summary

I need to get vulnerability reports for previous dates...

This pull request introduces a new --as-of command-line option to generate a vulnerability report from the perspective of a past date.

Testing

Run the command with a past date to see the historical state of vulnerabilities:

yarn repos --organization <YourOrg> --as-of 2023-01-01

Run the updated test suite to ensure all checks pass:

yarn test

[jeffsays]

thank you!