Add handling for details & summary tags (collapsible sections)

[kravhen]

Description

PR to solve #1422
Handles <details> and <summary> tags to allow for users to have collapsible sections in text.
Behavior is close to 1:1 with that of Github's handling.
Minor differences:

• on github, you need a blank line (\n\n) to have a proper newline in the resulting text. In this implementation, all newlines are handled (\n).
• on github, markdown inside <details> is not handled. In this implementation, it is.

Screenshots

The following text showing different structures will render as pictured on subsequent image:

Structure 1:

<details>no summary tags
lorem ipsum
and text on a new line</details>

----------

Structure 2:

<details>
<summary>text inside summary</summary>
lorem ipsum
</details>

----------

Structure 3:

<details>
    <summary>summary indentation single line</summary>
1. first thing
2. second thing 
3. third thing
</details>

----------

Structure 4:

<details>
  <summary>
    summary text here
  </summary>
  text inside details with *markdown* working **properly**
</details>

----------

Structure 5:

<details>
<summary>Shopping list</summary>
- Vegetables
- Fruits
- Fish
</details>

Additional Context

I was able to get it to work very close to github-like behavior without the additional imports of

import { fromMarkdown } from 'mdast-util-from-markdown'
import { toHast } from 'mdast-util-to-hast'

except I couldn't get inner lists and inner markdown to work, noting that markdown does not work inside <details> on github, but lists do.

Checklist

Are your changes backwards compatible? Please answer below:
Yes. Existing tags (though unlikely to have been used since they were not handled and would appear as plain text) will now render properly.

On a scale of 1-10 how well and how have you QA'd this change and any features it might affect? Please answer below:
7/10
Tested a variety of markdown inside <details> and various ways of structuring the tags: all tags on one line, a few lines, indentations, etc.
It works in bios.
Tags remain as plaintext in post titles as expected.
Will QA some more while in draft.

For frontend changes: Tested on mobile? Please answer below:
Yes. Works on mobile as expected.

Did you introduce any new environment variables? If so, call them out explicitly here:
No.

DRAFT

Creating this as draft for now for the following reasons:

• I would make a pass over styling to try to spruce it up a little visually, keeping consistent with the site's color scheme. I'm not a designer though so I'd try to make it slick but keep it subtle, simple.
• I can try to make newlines work as on github while keeping the markdown processing functionality, but am welcoming feedback if we rather it be truly 1:1 with behavior on github and forget markdown inside the tags.
• While merging this, I could also, if maintainers want, add a commit with the initial <spoiler> solution. That solution was little additional code and lean, leveraging the stylers already used for <sub> and <sup>. The use cases could be also be different. A spoiler for someone who wants to hide the winner of a sports match or a character dying in a show. The <details>/<summary> tags for breaking down bigger posts into smaller chunks or chapters, or also to hide answers like @SouthKoreaLN wished.

[kravhen]

I learned a lot about merging and rebasing, but the commit history is a helluva mess due to trying different things and forgetting other things since I first opened this.

Will submit a clean PR.