Decouple greentext extension from multi-line blockquotes and GFM alerts

Example:

```markdown
> First line.
> The second sentence in the first line.
> 
> Second line.
```

Expected output:

```html
<blockquote>
<p>
First line.
The second sentence in the first line.
</p>
<p>Second line.</p>
</blockquote>
```

As for gfm alerts, just the same:

```markdown
> [!Note]
> This is the first line.
> 
> Second line.
```
