Problem: mapping.lua can silently fall behind the ya2s/nonicons font
repo with no indication that new icons are available.
Solution: add a mapping-sync job that fetches nonicon.json from
upstream, generates the expected mapping.lua, and fails the check
if they differ.
Problem: nonicons.nvim lacks CI, pre-commit hooks, nix dev shell, issue
templates, and the README contains content that duplicates the vimdoc.
Solution: add quality/luarocks CI workflows, pre-commit config, nix
flake, prettier config, GitHub issue/discussion templates, and rewrite
the README to minimal style with all detailed content in vimdoc. Update
yamatsum/nonicons references to ya2s/nonicons.
