Don't allow use of the nullish coalescing operator in templates

[mollykreis]

Pull Request

🤨 Rationale

Resolves #1843

I did an evaluation of how observables work within FAST templates. A longer explanation of what is and is not allowed can be found in the discussion within the linked issue. The only issue I found within nimble code was the usage of ?? in templates, which may not correctly be identified as a volatile binding. Therefore, I've rewritten the bindings that previously used ?? and added a lint rule for template.ts files that disallows usage of ??.

👩‍💻 Implementation

See Rationale

🧪 Testing

All existing tests still pass
Verified that the new lint rule catches instances of ?? in template.ts files

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.