feat: initial nonicons.nvim rewrite

Problem: the original ya2s/nvim-nonicons fork diverged enough that
maintaining it as a fork was pointless. The set_icon() approach broke
with modern nvim-web-devicons, and the extensions directory coupled
integrations that belong in user config.

Solution: from-scratch rewrite with function wrapping + table mutation
override engine, vim.g.nonicons config pattern, vendored mapping with
integer codepoints, health check, vimdoc with recipes, and no built-in
plugin integrations.

plugin/nonicons.lua

@@ -0,0 +1,6 @@
+if vim.g.loaded_nonicons then
+  return
+end
+vim.g.loaded_nonicons = 1
+
+require('nonicons').apply()