Add client/server generation options

This patch splits the client and server generated code to two
files which should better support use of the client generated code in
the browser. Generation of the client/server code can also be
controlled via two plugin options:

```
--twirp_ts_opt=disable_client_generation # disables Twirp client code generation
--twirp_ts_opt=disable_server_generation # disables Twirp server code generation
```