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/util/io.go

@@ -9,6 +9,12 @@ import (
 	"io"
 )
 
+type NopCloser struct {
+	io.Writer
+}
+
+func (NopCloser) Close() error { return nil }
+
 // ReadAtMost reads at most len(buf) bytes from r into buf.
 // It returns the number of bytes copied. n is only less than len(buf) if r provides fewer bytes.
 // If EOF or ErrUnexpectedEOF occurs while reading, err will be nil.