This replaces Parcel with [esbuild][].
esbuild has a number of advantages over Parcel:
* It is much, much, *much* faster. A Parcel build takes several seconds on my system, while esbuild takes 60-70 _milliseconds_.
* Parcel hard-crashes when I switch branches on Windows, esbuild doesn't.
The one disadvantage is that, as far as I can tell, esbuild doesn't support any kind of auto-reloading in the browser when you change things. IMO this isn't that big a deal and the benefits outweigh this minor inconvenience (and anyways, auto-reload has historically backfired so many times that I usually have been manually reloading the page whenever I make changes).
[esbuild]: https://esbuild.github.io/