Export a proper public interface instead of a monolithic chunk

For cases where a consumer only needs a few functions instead of the entirety of `matrix`, like in https://github.com/matrix-org/matrix-react-sdk/pull/12871 - they shouldn't have to consume 1mb of JS to do simple things.

This may change the project more towards a library than an SDK though, and warrants team discussion.

Start by reviewing the issue context and the referenced matrix-react-sdk pull request, then inspect how matrix-js-sdk currently exposes its bundled functionality. The work is done when the team has defined and implemented a supported public interface that lets consumers use selected functions without consuming the entire bundle.

Written by the indexing model from the issue text.