feat: add deno target to rollup build script

[lsh]

This is a proof of concept to display a working build for #280.

Currently the Arquero distribution on Skypack does not work with Deno. This PR allows a cursory look into adding Deno as a distribution target. To see it in use, build Arquero normally (yarn build), then in a Deno REPL (assuming the current working directory is Arquero's root):

const aq = await import('./dists/arquero.deno.js');