Use esbuild instead

[LeoDog896]

(ref: chessground packaged files)

While I was digging around the chessground code, I noticed that the rollup build system isn't used for external purposes (not published to the NPM package) and barely serves a place internally. The max purpose it serves, rather, is that it tests chessground for recursive modules, and bundles for the internal demo.

If this is the case, it seems more fair to use esbuild as a check system rather than rollup.

[LeoDog896]

I need to rephrase this better. I'll be closing this in favor of the PR since the actual issue here is irrelevant.