chore(ci): Create web bundle for aztec.js

[spalladino]

For context, back in July a webpack step was added to the aztec.js build script, and the resulting main.js bundle was included in the package and exported for browser consumption. In v0.16.2, I believe here, the build:web step for aztec.js was removed, so 0.16.2 no longer had the main.js as part of it (0.16.1 still did). And later the browser entrypoint was removed altogether from the package.json.

It seems this web bundle was unused in web apps like the boxes, since those imported aztec.js and bundled them via their own webpack builds, which is how we expect most users to load apps. Still, aztecjs requires some polyfills that may not be straightforward to set up.

We should evaluate whether to ship a pre-built aztecjs bundle for the web, so users can directly import it from a page and use it directly, without any processing step needed.