Stop using const enums in the public API

[Feiyang1]

Some build tools (e.g. esbuild, vite) don't work with const enums because they compile Typescript in the isolatedModules mode. They compile files individually without type information, so they can't inline const enums, which eventually cause runtime errors.

Plus Javascript users can't use const enums.

Auth exports some const enums, e.g. ProviderId, SignInMethod. We should probably replace them with just plain object maps.

[BayanBennett]

Also, NextJS requires isolatedModules due to the way that they have babel configured.

Thank you for this change.