`Lib.run_hook_cmds` returns a table of tables (one for reach hook cmd
that was run). In `save_extra_cmds`, `vim.fn.writefile` expects a table
strings and if any of those strings contain newlines, they're
translated into nul characters. That means that there was no way to
return more than one line from a `save_extra_cmds` hook function.

To fix, we combine the results across all `save_extra_cmds` hooks and we
now support a `save_extra_cmds` hook returning either a string (possibly
with newlines) or table. If a hook returns a string then any newlines
will be used to split the string into individual line entries in the
table passed to `vim.fn.writefile`. If a hook returns a table, then each
of the values of the table will be added to the table passed to
`vim.fn.writefile` (the same newline splitting will also happen)

The code will already load it on demand as needed. If someone uses
Telescope, they'll already have it specified in their config so we don't
need to list it as a dependency