feat(ui): fold shows heading icons

[kkharji]

Purpose

Keep showing user customized heading icons in folds.

To achive same results:

vim.cmd[[hi Folded gui=bold guibg=NONE]]
vim.cmd[[set fillchars+=fold:\ ]]

[kkharji]

It would be cool to figure out how to keep the colors the same with/without folds. Also, it highlight the whole fold with something like the hack done with codeblocks "dark". But I think this can be for another PR. Unless, there's a easy solution

[vhyrro]

Looks great! I'll merge this as is right now. I might expand this later to also include other foldable elements because why not :)

As for the background highlight stuff to my understanding you can't do that since Folded is a single highlight group. You can't change the colour of an individual fold to my knowledge :(

Thanks again!