feat: add view.filter option

[epheien]

Because outline.nvim defaults to ignoring nobuflisted buffers, an option hook needs to be added for user customization.

After adding following options and 'epheien/outline-treesitter-provider.nvim' dependency, outline.nvim can support help (vimdoc) filetype.

{
  view = {
    filter = function(buf)
      local ft = vim.bo[buf].filetype
      if ft == 'qf' then return false end
      return vim.bo[buf].buflisted or ft == 'help'
    end,
  },
}

refer #34

[hedyhli]

I'm not sure adding support for vimdoc warrants a new configuration option to let the user customize the kinds of buffer to be filtered. It's better to make the default filter more lax, such as removing the check for buflisted altogether or special case for the help filetype. See: #76 (comment)

[hedyhli]

Thanks!

[epheien]

Removing the check for buflisted altogether or special case for the help filetype is simple.

But view.filter has an additional function, that is, when entering the buffer that is explicitly ignored, the outline will not clear the content.

And if there are some unlisted buffers that need to be supported in the future, the code needs to be modified each time.

[epheien]

Thanks!

Did you merge wrong PR? I just updated another PR #109.