Interactive markdown

[garlic0x1]

This adds interactive features to markdown code blocks, like how src blocks in org-mode work.

The API consists of 3 commands: markdown-eval-block, markdown-eval-block-and-insert, and markdown-kill-block-result, as well as one macro, register-block-evaluator for users to add support for other languages (lisp and bash work by default).

markdown-eval-block evaluates the source block your cursor is in, and outputs the result to a popup buffer (just like lisp-macroexpand)

markdown-eval-block-and-insert evaluates the block and inserts it's result below the block. If there was already a result there, it first deletes the old one.

markdown-kill-block-result deletes the result block below the code block your cursor is in, this is run automatically before markdown-eval-block-and-insert.

If you try to evaluate a block and there is no registered evaluator, a message is displayed saying so. If you use any of these commands outside a code block, nothing happens. If you use these commands while not in markdown-mode, a warning is displayed.

Evaluation commands work asynchronously and you can do whatever you want while it is working.

Here is a demo: Screencast from 2024-02-17 14-15-46.webm

[garlic0x1]

What is your guys' opinions on adding a feature like this that allows custom post-processing:

```http
GET http://localhost/json
```json

The idea is that this could be added on to output formatted JSON, this could be used in any situation where the ~a format is not desired. Examples include formatted JSON and HTML as well as using ~s or custom functions handling database tables.

Downside is that it doesn't look pretty in a markdown renderer, although for default behavior it would change nothing.

What are your thoughts?

Edit: prototype here https://github.com/garlic0x1/lem/tree/block-metadata