Add attention blocks within quote blocks for Note and Warning (#21711)

For each quote block, the first `**Note**` or `**Warning**` gets an icon
prepended to it and its text is colored accordingly. GitHub does this
(community/community#16925). [Initially requested on
Discord.](1038816475)

### Before

![image](https://user-images.githubusercontent.com/20454870/200408558-bd318302-6ff9-4d56-996f-9190e89013ec.png)

### After

![image](https://user-images.githubusercontent.com/20454870/200658863-1bac6461-dae7-4bf2-abd2-672d209574e4.png)

Signed-off-by: Yarden Shoham <hrsi88@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: silverwind <me@silverwind.io>

modules/markup/markdown/ast.go

@@ -180,3 +180,37 @@ func IsColorPreview(node ast.Node) bool {
 	_, ok := node.(*ColorPreview)
 	return ok
 }
+
+const (
+	AttentionNote    string = "Note"
+	AttentionWarning string = "Warning"
+)
+
+// Attention is an inline for a color preview
+type Attention struct {
+	ast.BaseInline
+	AttentionType string
+}
+
+// Dump implements Node.Dump.
+func (n *Attention) Dump(source []byte, level int) {
+	m := map[string]string{}
+	m["AttentionType"] = n.AttentionType
+	ast.DumpHelper(n, source, level, m, nil)
+}
+
+// KindAttention is the NodeKind for Attention
+var KindAttention = ast.NewNodeKind("Attention")
+
+// Kind implements Node.Kind.
+func (n *Attention) Kind() ast.NodeKind {
+	return KindAttention
+}
+
+// NewAttention returns a new Attention node.
+func NewAttention(attentionType string) *Attention {
+	return &Attention{
+		BaseInline:    ast.BaseInline{},
+		AttentionType: attentionType,
+	}
+}