The old `module.exports` syntax doesn't work downstream in the react-sdk or riot-web layers anymore, so this brings it up to 2019 standards. 

We do not use default exports anymore because they are discouraged by the ES6 community. It's not an official recommendation, but it's certainly commonplace. 

This also changes browser-index.js and index.js to use the `src` tree instead of `lib`. This is primarily to have IDEs autocomplete the package more easily, and because we're compiling this all in the riot-web layer it doesn't really matter. We still export `lib` and `dist/browser-matrix.js` for backwards compatibility (ie: everyone who has imported from `matrix-js-sdk/lib/whatever` or uses `<script>` tags). In a future version/PR we could drop these if we feel like it.

This commit does not address the test failures that are almost certainly caused by this change. While we're here, the ancient `"use strict";` flags have been removed as well.

Some files were not as easily converted. Most were simple keyword changes though there are a couple instances where the file structure is redone to accommodate ES6 exports.

Dist is where you Distribute things.