PLT-3985 - Show initial time for counterexamples in a human readable way

[palas]

This PR fixes a legacy vestige that caused the initial time of a counter example produced by the static analysis to be shown as a slot, even though it was an amount of milliseconds. So this PR changes the type names that referred to slots and displays the value as a date and a time.

It also updates the README.md that had one of the commands spelled wrong, and adds the scriv command to the nix development shell.

Pre-submit checklist:

• Branch
  • [ ] Tests are provided (if possible)
  • [x] Commit sequence broadly makes sense
  • [x] Key commits have useful messages
  • [x] Relevant tickets are mentioned in commit messages
  • [x] Formatting, materialized Nix files, PNG optimization, etc. are updated
• PR
  • [x] Self-reviewed the diff
  • [x] Useful pull request description
  • [x] Reviewer requested

[palas]

The code looks good, but I got this error while trying the static analysis... Not sure if related

...Failure: An error occurred while decoding a JSON value: Under 'Response': At object key 'result': Under 'Result': Under 'CounterExample': No value was found.

I cannot reproduce it. Have you run generate-purescript and the other commands in the README.md?