fix: some builtin functions missing in completion list (#2464)

if there is extension_list, then the builtin_list would be replaced/overriden. e.g. if there are 3 items in extension_list, then first 3 items in builtin_list would be replaced when you type
`:Telescope fi` and press tab to get completion list.

plugin/telescope.lua

@@ -118,7 +118,7 @@ end, {
     if n == 0 then
       return vim.tbl_filter(function(val)
         return vim.startswith(val, l[2])
-      end, vim.tbl_extend("force", builtin_list, extensions_list))
+      end, vim.tbl_flatten({builtin_list, extensions_list}))
     end
 
     if n == 1 then