chore: add jsr.json to .prettierignore

[mdjermanovic]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Ignores formatting of jsr.json files because release-please formats these files in a way that doesn't conform to prettier (example).

What changes did you make? (Give an overview)

Added jsr.json to .prettierignore.

Related Issues

CI failure https://github.com/eslint/rewrite/actions/runs/9684111978/job/26721093532

> eslint-rewrite@1.0.0 fmt:check
> prettier --check .

Checking formatting...
[warn] packages/core/jsr.json
[warn] Code style issues found in the above file. Run Prettier with --write to fix.
Error: Process completed with exit code 1.

Is there anything you'd like reviewers to focus on?