How should we represent enumerated types?

Stub issue, will fill out more detail as we prioritize.

Quick list of alternatives (probably not exhaustive):

1. TypeScript `enum`. Risky for a variety of reasons, which are [only growing](https://devblogs.microsoft.com/typescript/announcing-typescript-5-8-beta/#the---erasablesyntaxonly-option).
2. Bare union type (e.g. `'foo' | 'bar' | ...`)
3. Something `enum`-like, derived from plain JavaScript values ([some examples](https://github.com/getodk/web-forms/pull/288#discussion_r1931023931))
4. ?

Start by reviewing the four representation alternatives in the issue, including the linked TypeScript 5.8 note and the examples in pull request #288. Compare their implications for this project and record a decided approach with enough detail to guide later implementation.

Written by the indexing model from the issue text.