fix17697.dd

Fix Issue 17697 - Ddoc: automatically highlight URLs outside of macro arguments

URLs which appear in Ddoc text:

---
/****
 * http://www.fooa.com/test1
 */
---

will be treated as if they were written:

---
/****
 * $(LINK http://www.fooa.com/test1)
 */
---

and a clickable hyperlink will be emitted to the resulting .html file.

This detection does not happen if the URL is part of a macro argument:

---
/****
 * $(DOLLAR)$(LPAREN)ABC http://www.fooa.com/test1$(RPAREN)
 */
---

The behavior of that is unchanged. URLs can start with `http://` or `https://`

`http:` and `https:` at the beginning of the line are no longer treated
as section headings.