feat: configurable orderd list icons

[benlubas]

Allow the user to configure ordered list icons.

ex:

["core.concealer"] = {
  config = {
    ordered_icons = {
      ["1"] = function(i)
        if i < 10 then
          return "0" .. i
        end
        return tostring(i)
      end,
    },
  },
},

Yields:

---

This partially addresses a small issue where ordered lists that exceed 9 items in length, will not line up (b/c 10 is a char longer than 1). This allows people to pad single digits, but the padding will always be present.

Ideally the function above could take an index but also some context that includes the length of the list it's a part of. But this would require a more complex change