nvim: Use markdown ftplugin for djot as well

Simply import the markdown settings. We have to make use of the vim cmd
runtime (suggested here:
https://reddit.com/r/neovim/comments/ykpnob/require_one_ftpluginftlua_into_another/)
since lua requires cannot (afaik) exit the lua folder in the config
directory.

nvim/.config/nvim/after/ftplugin/djot.lua

@@ -0,0 +1,2 @@
+-- just copy markdown settings
+vim.cmd.runtime({"after/ftplugin/markdown.lua", bang=true})