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" />

README.md

@@ -236,6 +236,8 @@ Pass a table to the setup call with your configuration options.
     -- See :help 'winhl'
     -- To change background color to "CustomHl" for example, use "Normal:CustomHl".
     winhl = '',
+    -- Message displayed when there are no providers avialable.
+    no_provider_message = 'No supported provider...'
   },
 
   outline_items = {