feat(statusline): add custom separator between segments

[Kayzels]

Description

In the statusline function for the renderer, the code indicated that a separator should be able to be passed in, which would then separate the different components.

However, it was only ever called in the API statusline code, with status = renderer:statusline(). So, there was no way to add the separator.

This PR adds sep as an optional field for the API statusline code, and passes that value through to the renderer.

It also removes the trailing empty string that means that there was always the separator, even when there was only one item showing.

Screenshots

These were done with this config set for lualine (just the trouble.nvim winbar component)

{
    "nvim-lualine/lualine.nvim",
    opts = function(_, opts)
      local trouble = require("trouble")
      local symbols = trouble.statusline({
        mode = "symbols",
        groups = {},
        title = false,
        filter = { range = true },
        format = "{kind_icon}{symbol.name:Normal}",
        sep = "  ",
      })
      opts.winbar.lualine_b = {
        {
          symbols and symbols.get,
          cond = function()
            return vim.b.trouble_lualine ~= false and symbols.has()
          end,
        },
      }
    end
}