feat: Add configuration for no providers message

I am using outline.nvim as my permanent sidebar and I am constantly
annoyed by this message always displaying when I am openning any non lsp
related buffer e.g. picker or fugitive window

<img width="1335" alt="Image" src="https://github.com/user-attachments/assets/09384652-b8d0-4b57-8e0e-07a6e73b65fc" />

With this config I can set `no_provider_message` to empty string and
enjoy a clean sidebar if there is nothing to show:

<img width="1437" alt="Image" src="https://github.com/user-attachments/assets/f411b995-c0ec-4d06-980f-bad17f490bf5" />

lua/outline/sidebar.lua

@@ -371,7 +371,7 @@ end
 -- stylua: ignore end
 
 function Sidebar:no_providers_ui()
-  self.view:rewrite_lines({ 'No supported provider...' })
+  self.view:rewrite_lines({ cfg.o.outline_window.no_provider_message })
   vim.api.nvim_win_set_cursor(self.view.win, { 1, 0 })
 end