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.

nonicons.nvim-scm-1.rockspec

@@ -0,0 +1,21 @@
+rockspec_format = '3.0'
+package = 'nonicons.nvim'
+version = 'scm-1'
+
+source = {
+  url = 'git+https://github.com/barrettruth/nonicons.nvim.git',
+}
+
+description = {
+  summary = 'Nonicons for Neovim',
+  homepage = 'https://github.com/barrettruth/nonicons.nvim',
+  license = 'MIT',
+}
+
+dependencies = {
+  'lua >= 5.1',
+}
+
+build = {
+  type = 'builtin',
+}