This commit adds support for generated Typescript clients to be
compitable with [Next.JS](https://nextjs.org/) applications with the new `--nextjs` argument.

The existing client was not compiatble as we where using namespaces
to mimic the packages within the Encore application, however namespaces
are somewhat depcreated and not all transpliers support them; the key one
here being Babel (which Next.JS uses under the hood).

When turned on the new `--nextjs` flag, therefore prevents the code generator
from emitting namespaces, and instead it uses the package name as a prefix for
generated types; such as `$package_$struct`.

To make NextJS development easier, this flag also tells the TypeScript code generator
to generate [SWR](https://swr.vercel.app/) wrappers for all API's, while maintaining the
promise based functions if direct calls are still required (i.e. to perform actions).

If the original api call was:
```
const api = new Client()

const rsp: Promise<Data> = api.MySvc.MyAPI("segment", { name: "bob" })
```

The SWR wrapper will be:
```
const api = new Client() // this needs to be a singleton for the application instance

const rsp: SWRResponse<Data> = api.MySvc.useMyAPI("segment", { name: "bob" }, { refreshInternval: 1000 })
```