general options: add listchars back

It was in the bépo config file and is useful to detect trailing spaces
and nbsp chars.

lua/general-options.lua

@@ -60,3 +60,7 @@ vim.o.tw = 80
 vim.o.winwidth = 88
 vim.o.scrolloff = 10
 vim.cmd "set formatoptions-=t"
+
+-- Make some typographic chars visible
+vim.opt.listchars:append({nbsp = "·", trail = "¤", eol = '↲'})
+vim.o.list = true