feat: add project infrastructure and rewrite README

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.

.prettierrc

@@ -0,0 +1,9 @@
+{
+  "proseWrap": "always",
+  "printWidth": 80,
+  "tabWidth": 2,
+  "useTabs": false,
+  "trailingComma": "none",
+  "semi": false,
+  "singleQuote": true
+}