I've spent the last few days doing a significant amount of work trying to benchmark and improve Immer's performance, to address #1152 .

As part of that, I frequently needed to rerun the tests, but the current Jest + TS-Jest setup was annoyingly slow.

We switched over to use Vitest for the Redux repos a while back. I've been very happy with it, and part of that is that it seems to run our own tests noticeably faster.

I tried migrating Immer's tests to Vitest, and got them working. They also seem to run significantly faster. So, I'm submitting the Vitest migration as a PR in case you're interested.

Note that the work for this PR includes a lot of AI-generated code from Claude. This is a relatively new experiment for me, and I've done a lot of careful babysitting to keep it going in the right direction and tried to review and check the results.

Overall, the changes in this PR:

• Added Vitest config files, including a new reporter that tries to mimic the existing "skip obsolete snapshot changes" behavior
• Updated tests to work with Vitest:
  • Changed jest references to vi
  • Restructured patch.js, as the existing runPatchTest abstraction was resulting in Vitest thinking no tests were in the file. Created a similar runPatchTests method and updated all usages to call that method instead, and ensured that all tests are being run.
  • changed some describe blocks to test to ensure they're executed correctly
• Removed Jest config files