Allow user to supply their own get_icon function to set file icons and highlights

[devkvlt]

For example, add an option renderer.icons.glyphs.get_icon_func that expects a function that takes filename, extension and options(?) as input and returns an icon and a highlight group.

[alex-courtis]

That could work. I'd prefer not to expose the node, however we could clone most of the node's data.

Two callbacks might be preferable get_icon get_hl.

[alex-courtis]

#2415 Decorators may be enhanced to with a user callback.

[alex-courtis]

Tracking at #2948