refactor: minor cleanups

[TrevorBurnham]

Description

This PR applies some minor code streamlining:

• Hoist regionProps objects to module-level constants to avoid recreation on every render
• Remove redundant optional chaining in highlight/index.tsx where token.className is already checked in the ternary condition

How has this been tested?

I've run the server and confirmed that the demos render as expected.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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