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.

vim.toml

@@ -0,0 +1,33 @@
+[selene]
+base = "lua51"
+name = "vim"
+
+[vim]
+any = true
+
+[jit]
+any = true
+
+[bit]
+any = true
+
+[assert]
+any = true
+
+[describe]
+any = true
+
+[it]
+any = true
+
+[before_each]
+any = true
+
+[after_each]
+any = true
+
+[spy]
+any = true
+
+[stub]
+any = true