Bug: `dirvish#add_icon_fn` break help file highlighting

[brianhuster]

I find it hard to explain, so I will go straight to reproducing

1. git clone --depth=1 https://github.com/justinmk/vim-dirvish
2. Create a file repro.vim in current directory and add the following

set rtp+=./vim-dirvish
helptags ALL
call dirvish#add_icon_fn({p -> p[-1:] == '/' ? '📂' : '📄'})
runtime! plugin/dirvish.vim

3. Open Nvim with nvim -u repro.vim
4. Open Dirvish with :Dirvish
5. Open any help doc. Eg. :h dirvish
6. You will see something similar to what I see

Neovim version

NVIM v0.11.0-dev-1589+g71507281fb                                                                                                                                          
Build type: RelWithDebInfo                                                                                                                                                 
LuaJIT 2.1.1736781742