Add command to show single bookmark details

`/bookmark list` lists all bookmarks with its details.
`/bookmark list <jid>` shows the details of a single bookmark.

Implement https://github.com/profanity-im/profanity/issues/1558

tests/unittests/ui/stub_ui.c

@@ -907,6 +907,11 @@ cons_show_bookmarks(const GList* list)
     check_expected(list);
 }
 
+void
+cons_show_bookmark(Bookmark* item)
+{
+}
+
 void
 cons_show_disco_items(GSList* items, const char* const jid)
 {

tests/unittests/xmpp/stub_xmpp.c

@@ -487,6 +487,12 @@ bookmark_get_list(void)
     return mock_ptr_type(GList*);
 }
 
+Bookmark*
+bookmark_get_by_jid(const char* jid)
+{
+    return NULL;
+}
+
 char*
 bookmark_find(const char* const search_str, gboolean previous, void* context)
 {