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.

lua/nonicons/init.lua

@@ -0,0 +1,43 @@
+local M = {}
+
+---@class nonicons.Config
+---@field override boolean
+
+---@type nonicons.Config
+local default = {
+  override = true,
+}
+
+local initialized = false
+
+---@type nonicons.Config
+local config
+
+local function ensure_initialized()
+  if initialized then
+    return
+  end
+  local user = vim.g.nonicons or {}
+  vim.validate('nonicons', user, 'table')
+  vim.validate('nonicons.override', user.override, 'boolean', true)
+  config = vim.tbl_deep_extend('force', default, user)
+  initialized = true
+end
+
+M.mapping = require('nonicons.mapping')
+
+function M.get(name)
+  local code = M.mapping[name]
+  if code then
+    return vim.fn.nr2char(code)
+  end
+end
+
+function M.apply()
+  ensure_initialized()
+  if config.override then
+    require('nonicons.override').apply()
+  end
+end
+
+return M