feat: implement a neo-tree source

I think it could be cool to implement a neo-tree source so you could potentially have a side bar view of all your current tags.

This is not important nor do i believe it to be an imperative feature to be implemented, Just thought it could be an interesting idea so I'm creating this just to leave this as a note.

https://github.com/nvim-neo-tree/example-source

Start by reading the linked neo-tree example-source to understand the source interface, then inspect grapple.nvim's existing tag and display entry points. Done means a neo-tree source can present the user's current tags in a sidebar view, with the integration behavior verified in the plugin.

Written by the indexing model from the issue text.