feat: icon_fetcher adds symbol parameter

[epheien]

For example, access control information can be added at the icon location.
However, it is known that clangd currently does not return access spec information, while ctags can.

local access_icons = {
  public = '○',
  protected = '◉',
  private = '●',
}
    ...
    icon_fetcher = function(kindstr, bufnr, symbol) ---@diagnostic disable-line
      local cfg = require('outline.config')
      local icon = cfg.o.symbols.icons[kindstr].icon
      if symbol and symbol.access then
        return icon .. ' ' .. access_icons[symbol.access]
      end
      return icon
    end,
    ...

[hedyhli]

This is a great idea. Thanks! Could you update the readme to reflect this new parameter, and add an example for it such as the example you showed?

[epheien]

Done.

[hedyhli]

Alright. Thanks for adding this.