feat(picker.select): add option to show or hide indexes

[doums]

Hi, introduce the option show_idx to show (default) or hide the number indexes printed in front of each item in the picker of vim.ui.select.

Then we can disable the indexes:

opts.picker = {
  -- rest of the config
  sources = {
    select = {
      show_idx = false,
    },
  },

--  Or via vim.ui.select
vim.ui.select(items, {
  snacks = { show_idx = false }
}, callback)

This is useful for minimal UI for example, input hidden, no preview, just the title and the option list to navigate and select.