Make set-lookup-handlers! additive

Consecutive calls to set-lookup-handlers! would redefine *all* lookup
handlers, unsetting unspecified ones, so you were forced to redefine all
handlers, even if you only wanted to change one. No more. Its side
effects are now additive.

Also adds :async handler support, however, due to their nature, they
cannot fall back to other handlers (there's no reliable way to detect
they worked or not).

To get around this, write a blocking wrapper around the old async method
and register it as a non-async handler.

modules/lang/javascript/config.el

@@ -148,7 +148,7 @@
     (setq-default company-backends (delq 'company-tide (default-value 'company-backends))))
   (set-company-backend! 'tide-mode 'company-tide)
   ;; navigation
-  (set-lookup-handlers! 'tide-mode
+  (set-lookup-handlers! 'tide-mode :async t
     :definition #'tide-jump-to-definition
     :references #'tide-references
     :documentation #'tide-documentation-at-point)