Editor: Extend markdown to support HEEx features

Regular Markdown converter isn't capable of handling HEEx features. We want a subset of a regular Markdown that is capable of parsing and generating valid HEEx without losing features or breaking the compilation.

### Function components

Beacon provides function components to display assets and will provide components for building pages, for eg:

```elixir

Using such component should generate the expected HTML tags as it would in a HEEx template.

### Navigation

A regular markdown link should probably generate the same markup as `<.link navigate=...">` to include the `phx-*` attributes and allow it to transition pages efficiently, otherwise it triggers a redirect.

### Research

[MDX](https://mdxjs.com) is a great solution for the JS ecosystem and we it can serve as inspiration to build something similar in Elixir.

The library [md](https://hex.pm/packages/md) ~may be~ is [capable of embedding heex](https://elixirforum.com/t/implementing-custom-markdown-parser-with-the-md-library/55025) but it doesn't support the CommonMark spec.

So a new library [mdex](https://hex.pm/packages/mdex) was created to support what is needed for Beacon, but loading Phoenix Components is still a WIP.

The issue names no repository files, tests, or entry points. Start by reviewing the mdex and md libraries and linked pull request #76, then compare the proposed component and navigation behavior with the existing editor; done means Markdown can preserve the described HEEx features and produce valid, compiling output.

Written by the indexing model from the issue text.