fix(term_preview): bad bat command generation (nvim-telescope#3256)

Using `bat` would result in the command being a nested list.

eg. using `:Telescope plaents` with `bat` installed
```
{
  "bat",
  { "--pager", "less -RS" },
  "--style=plain",
  "--color=always",
  "--paging=always",
  "--",
  "/home/jt/projects/telescope.nvim/data/memes/planets/earth",
}
```

This would cause `vim.fn.termopen` to throw an error as the command is
expected to be a flat list or string.

lua/telescope/previewers/term_previewer.lua

@@ -37,18 +37,18 @@ local bat_maker = function(filename, lnum, start, finish)
   local command = { "bat" }
 
   if lnum then
-    table.insert(command, { "--highlight-line", lnum })
+    vim.list_extend(command, { "--highlight-line", lnum })
   end
 
   if has_less then
     if start then
-      table.insert(command, { "--pager", string.format("less -RS +%s", start) })
+      vim.list_extend(command, { "--pager", string.format("less -RS +%s", start) })
     else
-      table.insert(command, { "--pager", "less -RS" })
+      vim.list_extend(command, { "--pager", "less -RS" })
     end
   else
     if start and finish then
-      table.insert(command, { "-r", string.format("%s:%s", start, finish) })
+      vim.list_extend(command, { "-r", string.format("%s:%s", start, finish) })
     end
   end