Allow to use other glyphs for trees

Given that you intentionally are using just a handful of glyphs (as discussed in https://github.com/hlissner/emacs-doom-themes/issues/262) could you:

1. Just call propertize by hand instead of using the all-the-icons wrapper. This is a simple change, the wrapper is handy but it's not a big deal anyway.

2. Allow the user to provide a string/list/alist with the few glyphs you have to set.

I'm requesting this because many users probably have Nerd fonts or similar installed in their system and the dependency on all-the-icons is a source of potential conflicts, I guess. And even if it isn't it would be cleaner to provide 5 or 6 fonts without the need to install 6 font families first (btw, the all-the-icons installer is [broken](https://github.com/domtronn/all-the-icons.el/issues/134)).

I don't mean to remove all-the-icons support altogether, of course, it's a good fallback. Just to give a hook to users (probably the ones on arch linux) already using glyphs packages of their liking.