JavaScript client library doesn't work under Bun

[svix-jbrown]

The JS client library can't be imported from Bun, and the tests don't work in bun:

-------------------------------
1 | import { Svix } from "..";
2 | import type { SvixRequestContext } from "../request";
3 | 
4 | export class SvixInternal extends Svix {
                                      ^
ReferenceError: Cannot access 'Svix' before initialization.
      at svix-webhooks/javascript/src/api_internal/index.ts:4:35

The test suite doesn't work because bun requires that you have a describe() block and doesn't supported nested test calls.

Ideally at the end of this ticket, the library would work equally well under Node and Bun, and the test suite would run automatically under Bun.