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.

.luarc.json

@@ -0,0 +1,8 @@
+{
+  "runtime.version": "Lua 5.1",
+  "runtime.path": ["lua/?.lua", "lua/?/init.lua"],
+  "diagnostics.globals": ["vim", "jit"],
+  "workspace.library": ["$VIMRUNTIME/lua", "${3rd}/luv/library"],
+  "workspace.checkThirdParty": false,
+  "completion.callSnippet": "Replace"
+}