Switch from parcel to esbuild

[jlfwong]

https://github.com/evanw/esbuild now contains all of the necessary features for speedscope to switch to. Code splitting was the last remaining requirement, and that was landed recently.

There are still a number of open questions here, like how to serve the assets in development & how to get hot module reloading working.

I'm interested in switching for two major reasons:

• esbuild is really, really fast. I suspect that a full no-cache rebuild with esbuild is faster than an incremental rebuild with parcel
• parcel is a very large dependency in terms of # of transitive npm dependencies it brings in (59 deps as of writing). This make maintaining speedscope annoying because I get security advisories for transitive dependencies that I don't care about. It also makes cloning the repo & running npm install slower. esbuild has zero dependencies