`zindex` should apply to marks as well

[simonmandlik]

Currently, zindex applies only to the scrollbar "body", not to the marks displayed on it:

This means that any potential text is hidden. This is especially bad when using textwidth larger than window width and many marks are displayed, e.g. from gitsigns. This is a screenshot from a latex file:

This is with satellite disabled:

So many letters are hidden.

MWE:

vim.env.LAZY_STDPATH = ".repro"
load(vim.fn.system("curl -s https://raw.githubusercontent.com/folke/lazy.nvim/main/bootstrap.lua"))()

require("lazy.minit").repro({
  spec = {
    { "folke/flash.nvim", opts = {} },
    -- add any other plugins here
    { "lewis6991/satellite.nvim", opts = { zindex = 999 }}
  },
})