Pass function to filter_rules

[sarmong]

Problem:

There are far too many different utility windows created now - some examples include nvim-notify, fidget.nvim, alpha-nvim, nvim-tree, neotree, telescope, various scratchpads and many others. It is hard to keep track of all of them and add them to filter_rules of this plugin.

One solution would be to filter out all buffers with buftype=nofile and buflisted=false. However this falls short for the plugin like alpha-nvim, which has both of this set, however it is quite reasonable to want to open file replacing alpha in a window.

Proposed solution:

Allow users to pass a function that will receive bufnr (or winnr?) and will return true or false that will result in inclusion/exclusion of this window in window picking.