markup/goldmark: Add attributes support for blocks (tables etc.)

E.g.:

```
> foo
> bar
{.myclass}
```

There are some current limitations: For tables you can currently only apply it to the full table, and for lists the ul/ol-nodes only, e.g.:

```
* Fruit
  * Apple
  * Orange
  * Banana
  {.fruits}
* Dairy
  * Milk
  * Cheese
  {.dairies}
{.list}
```

Fixes #7548

markup/goldmark/internal/extensions/attributes/attributes.go

@@ -0,0 +1,119 @@
+package attributes
+
+import (
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
+)
+
+// This extenion is based on/inspired by https://github.com/mdigger/goldmark-attributes
+// MIT License
+// Copyright (c) 2019 Dmitry Sedykh
+
+var (
+	kindAttributesBlock = ast.NewNodeKind("AttributesBlock")
+
+	defaultParser                        = new(attrParser)
+	defaultTransformer                   = new(transformer)
+	attributes         goldmark.Extender = new(attrExtension)
+)
+
+func New() goldmark.Extender {
+	return attributes
+}
+
+type attrExtension struct{}
+
+func (a *attrExtension) Extend(m goldmark.Markdown) {
+	m.Parser().AddOptions(
+		parser.WithBlockParsers(
+			util.Prioritized(defaultParser, 100)),
+		parser.WithASTTransformers(
+			util.Prioritized(defaultTransformer, 100),
+		),
+	)
+}
+
+type attrParser struct{}
+
+func (a *attrParser) CanAcceptIndentedLine() bool {
+	return false
+}
+
+func (a *attrParser) CanInterruptParagraph() bool {
+	return true
+}
+
+func (a *attrParser) Close(node ast.Node, reader text.Reader, pc parser.Context) {
+}
+
+func (a *attrParser) Continue(node ast.Node, reader text.Reader, pc parser.Context) parser.State {
+	return parser.Close
+}
+
+func (a *attrParser) Open(parent ast.Node, reader text.Reader, pc parser.Context) (ast.Node, parser.State) {
+	if attrs, ok := parser.ParseAttributes(reader); ok {
+		// add attributes
+		var node = &attributesBlock{
+			BaseBlock: ast.BaseBlock{},
+		}
+		for _, attr := range attrs {
+			node.SetAttribute(attr.Name, attr.Value)
+		}
+		return node, parser.NoChildren
+	}
+	return nil, parser.RequireParagraph
+}
+
+func (a *attrParser) Trigger() []byte {
+	return []byte{'{'}
+}
+
+type attributesBlock struct {
+	ast.BaseBlock
+}
+
+func (a *attributesBlock) Dump(source []byte, level int) {
+	attrs := a.Attributes()
+	list := make(map[string]string, len(attrs))
+	for _, attr := range attrs {
+		var (
+			name  = util.BytesToReadOnlyString(attr.Name)
+			value = util.BytesToReadOnlyString(util.EscapeHTML(attr.Value.([]byte)))
+		)
+		list[name] = value
+	}
+	ast.DumpHelper(a, source, level, list, nil)
+}
+
+func (a *attributesBlock) Kind() ast.NodeKind {
+	return kindAttributesBlock
+}
+
+type transformer struct{}
+
+func (a *transformer) Transform(node *ast.Document, reader text.Reader, pc parser.Context) {
+	var attributes = make([]ast.Node, 0, 500)
+	ast.Walk(node, func(node ast.Node, entering bool) (ast.WalkStatus, error) {
+		if entering && node.Kind() == kindAttributesBlock && !node.HasBlankPreviousLines() {
+			attributes = append(attributes, node)
+			return ast.WalkSkipChildren, nil
+		}
+		return ast.WalkContinue, nil
+	})
+
+	for _, attr := range attributes {
+		if prev := attr.PreviousSibling(); prev != nil &&
+			prev.Type() == ast.TypeBlock {
+			for _, attr := range attr.Attributes() {
+				if _, found := prev.Attribute(attr.Name); !found {
+					prev.SetAttribute(attr.Name, attr.Value)
+				}
+			}
+		}
+		// remove attributes node
+		attr.Parent().RemoveChild(attr.Parent(), attr)
+	}
+}