Update to Parcel 2

[szhu]

Note to other people coming across this issue: The repo seems to be not actively maintained, so I'm not sure if this PR will get merged soon. If you want to use Parcel 2 now, feel free to use my fork:

https://github.com/szhu/electron-react-parcel-boilerplate

---

This is a PR that upgrades Parcel from v1 to v2.

In this PR I've changed as little as possible. I think this is the bare minimum set of changes required to get Parcel 2 to work. I'll add inline comments to explain each change I've made. Note, this is Parcel's official migration guide: https://parceljs.org/getting-started/migration/

I tested all the yarn scripts on a Mac and they seem to be working as intended. (All the apps build successfully, but I only tested that the Mac app runs successfully.)

Here are my version numbers

$ uname
Darwin

$ sw_vers
ProductName:	macOS
ProductVersion:	12.1
BuildVersion:	21C52

$ node --version
v16.13.1

$ yarn --version
1.22.17

Btw, thanks for making this repo! I'm just starting to learn Electron but I'm familiar with using Parcel and React, so this repo is incredibly helpful.