Dependency issue : service-worker-mock 'no longer maintained'

This template depends on `service-worker-mock`:

https://github.com/cloudflare/worker-typescript-template/blob/master/package.json#L32

The NPM package for this is found here:

https://www.npmjs.com/package/service-worker-mock

The Github repository link at NPM points here:

https://github.com/pinterest/service-workers

However, Github redirects requests to this repo to a new location:

https://github.com/zackargyle/service-workers

And this repo has had no changes since April 2020 and its README header now says:

```
*** THIS PROJECT IS NO LONGER MAINTAINED ***
```

This project should replace this abandoned dependency with one that is maintained.