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.

flake.nix

@@ -0,0 +1,29 @@
+{
+  description = "nonicons.nvim — nonicons for Neovim";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    systems.url = "github:nix-systems/default";
+  };
+
+  outputs =
+    {
+      nixpkgs,
+      systems,
+      ...
+    }:
+    let
+      forEachSystem = f: nixpkgs.lib.genAttrs (import systems) (system: f nixpkgs.legacyPackages.${system});
+    in
+    {
+      devShells = forEachSystem (pkgs: {
+        default = pkgs.mkShell {
+          packages = [
+            pkgs.prettier
+            pkgs.stylua
+            pkgs.selene
+          ];
+        };
+      });
+    };
+}