Refactor markup render system (#32533)

Remove unmaintainable sanitizer rules. No need to add special "class"
regexp rules anymore, use RenderInternal.SafeAttr instead, more details
(and examples) are in the tests

modules/markup/internal/finalprocessor.go

@@ -0,0 +1,30 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package internal
+
+import (
+	"bytes"
+	"io"
+)
+
+type finalProcessor struct {
+	renderInternal *RenderInternal
+
+	output io.Writer
+	buf    bytes.Buffer
+}
+
+func (p *finalProcessor) Write(data []byte) (int, error) {
+	p.buf.Write(data)
+	return len(data), nil
+}
+
+func (p *finalProcessor) Close() error {
+	// TODO: reading the whole markdown isn't a problem at the moment,
+	// because "postProcess" already does so. In the future we could optimize the code to process data on the fly.
+	buf := p.buf.Bytes()
+	buf = bytes.ReplaceAll(buf, []byte(` data-attr-class="`+p.renderInternal.secureIDPrefix), []byte(` class="`))
+	_, err := p.output.Write(buf)
+	return err
+}