vim.iter.flatten() is not a full replacement (does not support arrays
with `nil`); a fix may be backported but for now only use it on nightly
(0.10 has no warnings yet).
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.
