Allow instantiating Toucan before the request arrives

[leandro-manifesto]

Hello again.

So I've using Toucan for a while in a couple of Cloudflare Workers projects now and have been wondering if we could change it so that I could instantiate the instance only once in the global context and reuse it for every request, like this:

type Handler = (req: Request) => Response | Promise<Response>;

declare const SENTRY_DSN: string;

const sentry = new Toucan({ dsn: SENTRY_DSN });

async function catchErrors(event: FetchEvent, handler: Handler): Promise<Response> {
  try {
    return await handler(event.request);
  } catch (err) {
    sentry.captureException(err, event);

    return new Response(null, { status: 500 });
  }
}

Of course, this would be a breaking change, so another idea I had was to make an extends method:

type Handler = (req: Request) => Response | Promise<Response>;

declare const SENTRY_DSN: string;

const baseSentry = new Toucan({ dsn: SENTRY_DSN });

async function catchErrors(event: FetchEvent, handler: Handler): Promise<Response> {
  const sentry = baseSentry.extend({ event });

  try {
    return await handler(event.request);
  } catch (err) {
    sentry.captureException(err, event);

    return new Response(null, { status: 500 });
  }
}

These ways we could define defaults and avoid instantiating extra objects at every request, but since Toucan's constructor requires the FetchEvent, neither of these options are possible right now.