before:

```
kcab.worker.4bdc73a8d45eb2115156.js (2.1 MiB)
xfo.kcab.worker.4bdc73a8d45eb2115156.js (1010 KiB)
```

after:

```
kcab.worker.39f5fba82d7bc7477962.js (1.41 MiB)
xfo.kcab.worker.39f5fba82d7bc7477962.js (1000 KiB)
```

What’s changed:

- `loot-core` did not have a `browserslist` config, so
`@babel/preset-env` assumes we want to [transpile all the way back to
ES5](https://babeljs.io/docs/options#no-targets). I’ve removed the
`browserslist` config from each of the `package.json` files and moved it
to the root so this doesn’t happen again.
- I updated the target from `electron 3.0` to `electron 12.0` to match
our Electron dependency
- I’ve added `defaults` (currently equivalent to `> 0.5%, last 2
versions, Firefox ESR, not dead`) which is [recommended by
browserslist](https://browsersl.ist/#q=defaults). We could consider
tightening this, but it doesn’t offer a ton of space savings at this
point to just target Electron 12.
- Since much less transpilation will be happening, stack traces (dev and
prod) will be much easier to read!