Add org.thymeleaf.engine.IterationStatusVar

[exoego]

What does this PR do?

Add org.thymeleaf.engine.IterationStatusVar in Thymeleaf's reflect-config.json.

Ref. thymeleaf/thymeleaf#987

Code sections where the PR accesses files, network, docker or some external service

• No I/O, just the added unit test.

Checklist before merging

• I have considered including reachability metadata directly in the library or the framework (see our contributing document)
• I am the original author of all content provided in the pull request, and I did not copy the content from any other source (see the tests section from our contributing document)
• For all tests where I am not the sole author, I have added a comment that proves I may publish them under the specified license (see the tests section from our contributing document)
• I have properly formatted metadata files (see our formatting guide)
• I have added thorough tests (see the tests section from our contributing document)
• I have filled all places where my pull request accesses files, network, docker, or any other external service (see the section above)

[francis-a]

Thank you!