Upgrade dependencies for CVE-2020-7660

When using this project as a template, I was given security warnings concerning https://github.com/advisories/GHSA-hxcc-f52p-wc94

What I did to fix this after generating a new project from this template:

mochajs/mocha#4375
- update https://github.com/cloudflare/worker-typescript-template/blob/9a1574a532bcdd3da55e5b19e5d9fb57df4078c2/package.json#L21

```
"mocha": "^8.1.0",
```

webpack/webpack#11309
- https://github.com/webpack/webpack/issues/11309#issuecomment-674128535

Afterwards, the example `npm test` and `npm run build` succeeded.