Generate tagged enums in C# client code

[RReverser]

Description of Changes

Now that we use a shared BSATN runtime with C# modules, we can leverage our special TaggedEnum construct to support tagged enums in the client as well.

As a drive-by fix, also slightly simplifies TypeScript tagged enum output and marked C# TaggedEnum base record as abstract (since it shouldn't be constructed).

API and ABI breaking changes

If this is an API or ABI breaking change, please apply the
corresponding GitHub label.

Expected complexity level and risk

1

How complicated do you think these changes are? Grade on a scale from 1 to 5,
where 1 is a trivial change, and 5 is a deep-reaching and complex change.

This complexity rating applies not only to the complexity apparent in the diff,
but also to its interactions with existing and future code.

If you answered more than a 2, explain what is complex about the PR,
and what other components it interacts with in potentially concerning ways.

Testing

Describe any testing you've done, and any testing you'd like your reviewers to do,
so that you're confident that all the changes work as expected!

• Ran codegen tests and updated snapshots.

[gefjon]

Thanks! For posterity, I asked Ingvar to do this because it's useful for #1077 , as it will allow us to use the CLI to codegen the websocket bindings.