v5: Add Parcel support

[mdo]

Drops most of the custom npm scripts for Parcel. Still needs some fine-tuning and testing, but I'm liking this setup (much less complicated than Webpack based on what I've run into).

• [ ] Double check if I can include Icons another way
• [ ] Audit package-lock
• [ ] Verify readme updates
• [ ] Update HTML page
• [ ] Update CI and package.json scripts
• [ ] Verify things work on native Windows too