Make most features work in CommonMark mode: head, live code blocks, tabs...

### Motivation

Related to the CommonMark support issue: https://github.com/facebook/docusaurus/issues/3018

Some custom elements we created do not work when using the CommonMark parser mode, that you can turn on using:
- `frontMatter.mdx.format: 'md'`
- `config.markdown.format: 'detect'` + using the `.md` extension

---

I think it would be convenient for users to have features working fine in both modes in a consistent way. I believe it is possible, but may require some adaptations.

Here are some components that do not currently work:

---

## SEO

This should work in CommonMark too, but currently does not:

```md

HEAD Markdown Page tests title

```

## Summary/details

Renders as:


Instead of:


---

## Live code editor

Renders as:


Instead of:


---

## Tabs

Renders as: N/A because we don't have any syntax that does not require using MDX/ESM imports.

Instead of:


Possible solution? IMHO this syntax should be allowed without imports:

```md

This is an apple 🍎


This is an orange 🍊


This is a banana 🍌

```

### Structured data

As part of https://github.com/facebook/docusaurus/pull/9669 we are adding a `@theme/StructuredData` helper component:

```mdx
import StructuredData from "@theme/StructuredData";

# Title
```

We should figure out how to define structured data in CommonMark mode

### Self-service

- [X] I'd be willing to do some initial work on this proposal myself.