Add custom elements manifest

[justinfagnani]

It'd be great if this had a manifest so it'd show up in future catalogs and have intellisence in IDEs.

See https://validator.elematic.dev/validate?packageName=cally

[WickyNilliams]

Yes! I thought i already had an issue for this, so thanks for raising one. It's been on my mind since the beginning.

Atomico does have a CEM plugin, but it's not up to date with their latest syntax. I did start hacking away on a fix but it was turning to a time sink before I first released Cally. Never got back around to it.

I have started hacking away on migrating to vanilla WCs mostly for file size savings, but also because it'll give me CEM analyzer support for free