Prepare for Goldmark

This commmit prepares for the addition of Goldmark as the new Markdown renderer in Hugo.

This introduces a new `markup` package with some common interfaces and each implementation in its own package.

See #5963

hugolib/site_test.go

@@ -1018,6 +1018,7 @@ func TestRefLinking(t *testing.T) {
 }
 
 func checkLinkCase(site *Site, link string, currentPage page.Page, relative bool, outputFormat string, expected string, t *testing.T, i int) {
+	t.Helper()
 	if out, err := site.refLink(link, currentPage, relative, outputFormat); err != nil || out != expected {
 		t.Fatalf("[%d] Expected %q from %q to resolve to %q, got %q - error: %s", i, link, currentPage.Path(), expected, out, err)
 	}