api: Add FUNC_ATTR_DEFERRED attribute to a number of functions

Any function that can directly mutate the screen or execute vimscript had the
attribute applied.

src/nvim/api/tabpage.c

@@ -62,6 +62,7 @@ Object tabpage_get_var(Tabpage tabpage, String name, Error *err)
 /// @param[out] err Details of an error that may have occurred
 /// @return The tab page handle
 Object tabpage_set_var(Tabpage tabpage, String name, Object value, Error *err)
+  FUNC_ATTR_DEFERRED
 {
   tabpage_T *tab = find_tab_by_handle(tabpage, err);