refactor: convert to ES Modules and remove traces of CommonJS except in Rollup config because some dependencies are still CommonJS

BREAKING: The new project layout might break in some tooling setups.

We've added an exports field to `package.json` to specify where
statements like `import ... from 'docsify'` will import from, and left
the `main` and `unpkg` fields as-is for backwards compatibility with the
global <script> import method. Most people who use a non-module
`<script>` tag to import Docsify will not notice a difference. Anyone
else who is importing Docsify into a specilized build setup using
`import` statements has a chance of being broken, so we've marked this
as BREAKING.