chore: use importmap/esm.sh in demo

[jgravois]

i'm not sure why, but https://cdn.skypack.dev/arc@1 still doesn't resolve.

all these work 🤷
https://cdn.skypack.dev/arc
https://cdn.skypack.dev/arc@1.0.0
https://cdn.skypack.dev/arc@latest

this PR swaps in esm.sh and importmap syntax instead.

ref: skypackjs/skypack-cdn#365

[springmeyer]

No concern switching to something like esm, but just want to gut check: is using a cdn valuable enough to keep one or should we just build the arc.js into the bundle and load it that way?

[jgravois]

should we just build arc.js into the bundle

now that both browsers and node support ESM natively, my take is that the maintenance overhead outweighs the benefit.

using a CDN like this makes the demo 'plug n play' and points at the exact same code folks would be installing themselves via npm install arc.

caveat: esm.sh is doing a little magic behind the scenes to flatten things out into a single file. swapping in https://unpkg.com/arc@1 would load exactly what is published on npm.

[springmeyer]

Sound good, thanks for the explanation.