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.

stylua.toml

@@ -0,0 +1,8 @@
+column_width = 100
+line_endings = "Unix"
+indent_type = "Spaces"
+indent_width = 2
+quote_style = "AutoPreferSingle"
+call_parentheses = "Always"
+[sort_requires]
+enabled = true