TypeScript client SDK: improve usability of generated enum constants

[gefjon]

As requested on Discord.

Apparently, enum constants are being generated typed at { tag: string }, but their uses (e.g. reducer arguments) are typed more specifically at e.g. { tag: 'MyVariant' }. The Discord user suggests adding an as const to the definition of the constants. A manual type ascription might (or might not) also work.