feat: new load DDS event

[sxyazi]

Closes #1381

Closes #1852

---

This DDS event is used to subscribe to the directory loading state:

ps.sub("load", function(body)
  ya.err("tab:", body.tab)
  ya.err("path:", tostring(body.url))
  ya.err("is_loading:", body.stage.is_loading)
  ya.err("is_loaded:", not body.stage.is_loading)
end)

logs:

"tab:" 1
"path:" "/tmp/test-folder"
"is_loading:" false
"is_loaded:" true