Enable Element Call to be consumed as a package within a larger app

[robintown]

If we want to add Element Call as a dependency of Element Web and package it with the application, we should make yarn pack produce a built copy. Then it can be added as a simple Git dependency to projects using any modern package manager like Yarn Berry or NPM, and it will be saved and built in the local package cache.

Unfortunately, we don't use modern package managers yet for our web applications, we use Yarn Classic. This is in draft while I ask around about whether now is a good time to change that.

Part of element-hq/element-web#23908

[hughns]

I've been cribbing bits of this for #2994 and some bits have already landed.

Things I've done:

• using --frozen-lockfile in CI
• using the embedded approach that you took and relocating index.html

The one thing I haven't used is the build during yarn install. Instead we only need to do a build for releases of Element Call.

Thank you!