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/markup_config/config.go

@@ -44,6 +44,8 @@ type Config struct {
 func Decode(cfg config.Provider) (conf Config, err error) {
 	conf = Default
 
+	normalizeConfig(cfg)
+
 	m := cfg.GetStringMap("markup")
 	if m == nil {
 		return
@@ -65,6 +67,17 @@ func Decode(cfg config.Provider) (conf Config, err error) {
 	return
 }
 
+func normalizeConfig(cfg config.Provider) {
+	// Changed from a bool in 0.81.0
+	const attrKey = "markup.goldmark.parser.attribute"
+	av := cfg.Get(attrKey)
+	if avb, ok := av.(bool); ok {
+		cfg.Set(attrKey, goldmark_config.ParserAttribute{
+			Title: avb,
+		})
+	}
+}
+
 func applyLegacyConfig(cfg config.Provider, conf *Config) error {
 	if bm := cfg.GetStringMap("blackfriday"); bm != nil {
 		// Legacy top level blackfriday config.