Manage browser binaries directly instead of letting puppeteer do it

Since we depend on `puppeteer`, when someone installs `test-mule`, the `puppeteer` package will also be installed, which has a post-install script that causes the browser binaries to be downloaded into `node_modules`. If we instead use [`puppeteer-core`](https://github.com/puppeteer/puppeteer/blob/main/docs/api.md#puppeteer-vs-puppeteer-core), then puppeteer won't install the browser binaries for us, and we can handle downloading them ourselves. We can cache them in a global location, so that repeated `npm ci`'s don't cause the browser binaries to be re-downloaded every time.

Puppeteer has an API we can use to help manage browser binaries: https://github.com/puppeteer/puppeteer/blob/main/docs/api.md#class-browserfetcher