feat(completion): insert current company-mode selection with tab

THis is in addition to the RET keybinding.

modules/completion/siren-company.el

@@ -13,7 +13,8 @@
   ;; Scroll through company suggestions with C-n and C-p.
   (:map company-active-map
               ("C-n" . company-select-next)
-              ("C-p" . company-select-previous))
+              ("C-p" . company-select-previous)
+              ("<tab>" . company-complete-selection))
 
   :custom
   (company-begin-commands '(self-insert-command))