Feature: support for Markdown directive element syntax extension

This is a way of extending Markdown which is a bit alternative to direct use of HTML elements in Markdown and a bit more native to Markdown

https://talk.commonmark.org/t/generic-directives-plugins-syntax/444 (it has still proposal status, but is fairly stable for years). There also exists a stable remark plugin for parsing: https://github.com/remarkjs/remark-directive (which goes well with Lexical's new support for remark)

It has syntax of sort of (and fully customizable)

```markdown
:::div{.custom-class}

...

:::
```

Lexical might represent the directive syntax as a block and allow to collapse/expand its contents

Start by reviewing Lexical’s new remark support alongside the remark-directive plugin and the generic directives proposal linked in the issue. Determine how the example :::div{.custom-class} syntax should map into Lexical; done means directives are represented as blocks whose contents can be collapsed or expanded.

Written by the indexing model from the issue text.