fix: allow usage of keywords in relationDef

[ewanharris]

Description

This fixes the usage of keywords in the relationDef in a DSL, it adds to the existing tests but I'm not 100% sure if there's some avenues I might have missed (from and # maybe?)

It might be worth cherry-picking this over to the main branch (minus module/extend?) to fix there?

References

Review Checklist

• [x] I have clicked on "allow edits by maintainers".
• [ ] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• [x] The correct base branch is being used, if not main
• [ ] I have added tests to validate that the change in functionality is working as expected

[ewanharris]

Need to merge #195 first for the Go tests to run OK