html: Improve link abbreviation (#+LINK) resolution

Handle `%s`/`%h` interpolation and add support for shorthand of prefix only
links / links with empty tag (i.e. `[example]` instead of [example:]). See
[1] and [2]. As so often we don't match org mode output completely but our
output looks sane enough to me so let's move along for now.

[1] https://orgmode.org/manual/Link-Abbreviations.html
[2] https://github.com/bzg/org-mode/blob/main/lisp/ol.el#L1011

org/keyword.go

@@ -63,7 +63,7 @@ func (d *Document) parseKeyword(i int, stop stopFn) (int, Node) {
 	case "INCLUDE":
 		return d.parseInclude(k)
 	case "LINK":
-		if parts := strings.Split(k.Value, " "); len(parts) >= 2 {
+		if parts := strings.SplitN(k.Value, " ", 2); len(parts) == 2 {
 			d.Links[parts[0]] = parts[1]
 		}
 		return 1, k