Support GitHub-style alert/callout syntax (> [!NOTE], [!WARNING], etc.)

[YounesJY]

Add support for rendering GitHub-style alert blockquotes in the live preview — the > [!NOTE], > [!TIP], > [!IMPORTANT], > [!WARNING], > [!CAUTION] syntax that GitHub, GitLab, and Obsidian already render as styled, colored callout boxes.

Why
This syntax is now a de facto standard for adding semantic emphasis (notes, warnings, tips) to Markdown docs and READMEs. Right now MarkText treats these as plain blockquotes with no visual distinction, so there's no way to preview how they'll actually look once pushed to GitHub. Apps like Typora already support admonition-style callouts natively.

Proposed scope
Start with the core 5 GitHub-compatible types for maximum interoperability:

> [!NOTE]
> Useful info.

> [!TIP]
> Helpful advice.

> [!IMPORTANT]
> Key info.

> [!WARNING]
> Needs attention.

> [!CAUTION]
> Risk warning.

Each would render with a distinct color/icon in the live preview, while still saving as plain, portable Markdown blockquote syntax underneath.

Related
There's a related issue (#5223) proposing a larger Obsidian-style superset with foldable callouts ([!TYPE]+/[!TYPE]-) and more types. That's a great stretch goal, but I think shipping the simpler GitHub-compatible baseline first would deliver most of the value sooner and keep scope/interoperability tight. Curious what maintainers and others think on sequencing.

[Jocs]

I hope to support this feature in version 0.21.0

[Jocs]

Some thoughts about this feature:

1. Add GitHub Alerts to the paragraph menu, with a submenu for selecting one of the five types: NOTE, TIP, IMPORTANT, WARNING, and CAUTION. No keyboard shortcuts are required.
2. Add support for all five GitHub Alert types to Quick Insert.
3. Support converting a blockquote into a GitHub Alert by typing [!TYPE] followed by Enter, where TYPE is one of the five supported types.
4. Support converting between regular paragraphs, blockquotes, and different GitHub Alert types using the paragraph menu.
5. Match GitHub's rendering styles for GitHub Alerts in the editor and when exporting to HTML or PDF.
6. Add a configuration option with a default value of false. GitHub Alerts should only be rendered in the editor and included in HTML/PDF exports when this option is set to true. The GitHub Alerts option should also only appear in the paragraph menu when this setting is enabled.

[Jocs]

There's one point that seems slightly inconsistent with the description: GitHub Alerts shouldn't support nesting, right?

[YounesJY]

I'm not sure, I just want a proper support for that basic feature first as a MUST HAVE

The nesting can be delayed for another update.The attached image is just the only I had at that time.

[Jocs]

#5765 The PR has been submitted. Testing is welcome.

[Jocs]

Released in 0.22.0

[YounesJY]

Thanks again

Finally I'll have better MDs for my studies.