Enable development builds of the app using esbuild rather than Webpack

[shirakaba]

Not too sure what would be involved with this, but may not be out of reach.

[shirakaba]

I read that esbuild's plugins API is not stable yet, so using esbuild with Svelte would be brave. I'll wait at least until SvelteKit is out, which I believe may give some indication of how to integrate esbuild, rather than making any uninformed guesses myself.