fix(markdown): Fix GFM rendering if blank line right after magic keyword

[queengooborg]

Summary

This PR is a follow-up to #10168, which fixes the rendering of GFM noteblocks if there is a blank line after the magic keyword (which is required for callouts due to Prettier formatting).

Problem

See summary

Solution

The solution is simple: update the regex to remove the magic keyword from rendering to make the newline character afterwards optional, since a blank line in between the magic keyword and content generates multiple paragraphs instead of a newline character. Afterwards, any preceding blank lines are stripped for pretty rendering.

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Screenshots

Test Markdown used:

> [!NOTE]
> Hello!

> [!WARNING]
> Hello!

> [!CALLOUT]
> Hello!

---

> [!NOTE]
>
> Hello!

> [!WARNING]
>
> Hello!

> [!CALLOUT]
>
> Hello!

Before

After