Improve shortcode indentation handling

* Record the leading whitespace (tabs, spaces) before the shortcode when parsing the page.
* Apply that indentation to the rendered result of shortcodes without inner content (where the user will apply indentation).

Fixes #9946

common/text/transform.go

@@ -61,3 +61,17 @@ func Puts(s string) string {
 	}
 	return s + "\n"
 }
+
+// VisitLinesAfter calls the given function for each line, including newlines, in the given string.
+func VisitLinesAfter(s string, fn func(line string)) {
+	high := strings.Index(s, "\n")
+	for high != -1 {
+		fn(s[:high+1])
+		s = s[high+1:]
+		high = strings.Index(s, "\n")
+	}
+
+	if s != "" {
+		fn(s)
+	}
+}