Refactor markup package (#32399)

To make the markup package easier to maintain:
1. Split some go files into small files
2. Use a shared util.NopCloser, remove duplicate code
3. Remove unused functions

modules/markup/markdown/goldmark.go

@@ -45,7 +45,7 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 	ctx := pc.Get(renderContextKey).(*markup.RenderContext)
 	rc := pc.Get(renderConfigKey).(*RenderConfig)
 
-	tocList := make([]markup.Header, 0, 20)
+	tocList := make([]Header, 0, 20)
 	if rc.yamlNode != nil {
 		metaNode := rc.toMetaNode()
 		if metaNode != nil {

modules/markup/markdown/toc.go

@@ -7,13 +7,19 @@ import (
 	"fmt"
 	"net/url"
 
-	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/translation"
 
 	"github.com/yuin/goldmark/ast"
 )
 
-func createTOCNode(toc []markup.Header, lang string, detailsAttrs map[string]string) ast.Node {
+// Header holds the data about a header.
+type Header struct {
+	Level int
+	Text  string
+	ID    string
+}
+
+func createTOCNode(toc []Header, lang string, detailsAttrs map[string]string) ast.Node {
 	details := NewDetails()
 	summary := NewSummary()
 

modules/markup/markdown/transform_heading.go

@@ -13,14 +13,14 @@ import (
 	"github.com/yuin/goldmark/text"
 )
 
-func (g *ASTTransformer) transformHeading(_ *markup.RenderContext, v *ast.Heading, reader text.Reader, tocList *[]markup.Header) {
+func (g *ASTTransformer) transformHeading(_ *markup.RenderContext, v *ast.Heading, reader text.Reader, tocList *[]Header) {
 	for _, attr := range v.Attributes() {
 		if _, ok := attr.Value.([]byte); !ok {
 			v.SetAttribute(attr.Name, []byte(fmt.Sprintf("%v", attr.Value)))
 		}
 	}
 	txt := v.Text(reader.Source()) //nolint:staticcheck
-	header := markup.Header{
+	header := Header{
 		Text:  util.UnsafeBytesToString(txt),
 		Level: v.Level,
 	}