Refactor markdown render (#30139)

Only split the file into small ones (and rename AttentionTypes to
attentionTypes)

(cherry picked from commit 71706126b56616750a65290460fd211b9b8449da)
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

Conflicts:
	- modules/markup/markdown/goldmark.go
	- modules/markup/markdown/transform_blockquote.go
	Conflicts were resolved by favouring the Forgejo implementation:
	I copied the Forgejo code to the same place Gitea copied them
	to, and adjusted the imports accordingly.

	Apart from conflict resolution, this also moves
	`applyElementDir` from a local func in `goldmark.Transform` to a
	method on `*ASTTransformer`, to make it callable from the
	extracted functions.

modules/markup/markdown/prefixed_id.go

@@ -0,0 +1,59 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markdown
+
+import (
+	"bytes"
+	"fmt"
+
+	"code.gitea.io/gitea/modules/container"
+	"code.gitea.io/gitea/modules/markup/common"
+
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/util"
+)
+
+type prefixedIDs struct {
+	values container.Set[string]
+}
+
+// Generate generates a new element id.
+func (p *prefixedIDs) Generate(value []byte, kind ast.NodeKind) []byte {
+	dft := []byte("id")
+	if kind == ast.KindHeading {
+		dft = []byte("heading")
+	}
+	return p.GenerateWithDefault(value, dft)
+}
+
+// GenerateWithDefault generates a new element id.
+func (p *prefixedIDs) GenerateWithDefault(value, dft []byte) []byte {
+	result := common.CleanValue(value)
+	if len(result) == 0 {
+		result = dft
+	}
+	if !bytes.HasPrefix(result, []byte("user-content-")) {
+		result = append([]byte("user-content-"), result...)
+	}
+	if p.values.Add(util.BytesToReadOnlyString(result)) {
+		return result
+	}
+	for i := 1; ; i++ {
+		newResult := fmt.Sprintf("%s-%d", result, i)
+		if p.values.Add(newResult) {
+			return []byte(newResult)
+		}
+	}
+}
+
+// Put puts a given element id to the used ids table.
+func (p *prefixedIDs) Put(value []byte) {
+	p.values.Add(util.BytesToReadOnlyString(value))
+}
+
+func newPrefixedIDs() *prefixedIDs {
+	return &prefixedIDs{
+		values: make(container.Set[string]),
+	}
+}