refactor: extract Antimony keywords into AntimonyConstants interface

[dyrpsf]

Description

Following up immediately on @draeger's suggestion at the end of #310.

This PR extracts all Antimony-specific keywords, operators, and formatting strings into a centralized AntimonyConstants interface. AntimonySerializer now implements this interface, inheriting the constants directly.

This removes all hardcoded magic strings from the serializer logic, making the parser significantly cleaner and safer to maintain moving forward.

All core tests (AntimonySerializerTest.java) continue to pass cleanly!

[Schmoho]

Here too I would propose to address #313

[dyrpsf]

I just pushed AntimonyConstantsTest.java to ensure the interface variables are properly verified against regressions! Let me know if everything looks good to go.

[dyrpsf]

Just pushed a quick follow-up commit to add the proper Javadoc and @author tags to the new test file so it perfectly matches the repository's documentation standards!