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.](https://discord.com/channels/322538954119184384/322538954119184384/1038816475638661181)

### 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,
+	}
+}

modules/markup/markdown/goldmark.go

@@ -14,6 +14,7 @@ import (
 	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/markup/common"
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/svg"
 	giteautil "code.gitea.io/gitea/modules/util"
 
 	"github.com/microcosm-cc/bluemonday/css"
@@ -46,6 +47,7 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 		ctx.TableOfContents = make([]markup.Header, 0, 100)
 	}
 
+	attentionMarkedBlockquotes := make(container.Set[*ast.Blockquote])
 	_ = ast.Walk(node, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
 		if !entering {
 			return ast.WalkContinue, nil
@@ -184,6 +186,18 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 			if css.ColorHandler(strings.ToLower(string(colorContent))) {
 				v.AppendChild(v, NewColorPreview(colorContent))
 			}
+		case *ast.Emphasis:
+			// check if inside blockquote for attention, expected hierarchy is
+			// Emphasis < Paragraph < Blockquote
+			blockquote, isInBlockquote := n.Parent().Parent().(*ast.Blockquote)
+			if isInBlockquote && !attentionMarkedBlockquotes.Contains(blockquote) {
+				fullText := string(n.Text(reader.Source()))
+				if fullText == AttentionNote || fullText == AttentionWarning {
+					v.SetAttributeString("class", []byte("attention-"+strings.ToLower(fullText)))
+					v.Parent().InsertBefore(v.Parent(), v, NewAttention(fullText))
+					attentionMarkedBlockquotes.Add(blockquote)
+				}
+			}
 		}
 		return ast.WalkContinue, nil
 	})
@@ -273,6 +287,7 @@ func (r *HTMLRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
 	reg.Register(KindSummary, r.renderSummary)
 	reg.Register(KindIcon, r.renderIcon)
 	reg.Register(ast.KindCodeSpan, r.renderCodeSpan)
+	reg.Register(KindAttention, r.renderAttention)
 	reg.Register(KindTaskCheckBoxListItem, r.renderTaskCheckBoxListItem)
 	reg.Register(east.KindTaskCheckBox, r.renderTaskCheckBox)
 }
@@ -309,6 +324,28 @@ func (r *HTMLRenderer) renderCodeSpan(w util.BufWriter, source []byte, n ast.Nod
 	return ast.WalkContinue, nil
 }
 
+// renderAttention renders a quote marked with i.e. "> **Note**" or "> **Warning**" with a corresponding svg
+func (r *HTMLRenderer) renderAttention(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	if entering {
+		_, _ = w.WriteString(`<span class="attention-icon attention-`)
+		n := node.(*Attention)
+		_, _ = w.WriteString(strings.ToLower(n.AttentionType))
+		_, _ = w.WriteString(`">`)
+
+		var octiconType string
+		switch n.AttentionType {
+		case AttentionNote:
+			octiconType = "info"
+		case AttentionWarning:
+			octiconType = "alert"
+		}
+		_, _ = w.WriteString(string(svg.RenderHTML("octicon-" + octiconType)))
+	} else {
+		_, _ = w.WriteString("</span>\n")
+	}
+	return ast.WalkContinue, nil
+}
+
 func (r *HTMLRenderer) renderDocument(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
 	n := node.(*ast.Document)