feat(nix): add flake.nix (#13487)

`flakes` is a feature available in the unstable version of the `nix` package manager
to ease the distribution of nix expressions see https://www.tweag.io/blog/2020-05-25-flakes/ .

Nix excels at software reproducibility and is available/can be
installed on different distributions (ubuntu/void/debian etc) as well
as macOS.

This flakes contains 3 packages that can be installed via for instance
`nix run 'github:neovim/neovim?dir=contrib#nvim-debug'`
or
`nix run 'github:neovim/neovim?dir=contrib'` for the default version.

you can register an alias
`nix registry add neovim 'github:neovim/neovim?dir=contrib'`
and the previous commands become:
`nix run neovim`

contrib/flake.nix

@@ -0,0 +1,95 @@
+{
+  description = "Neovim flake";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }: let
+    system = "x86_64-linux";
+    legacyPkgs = nixpkgs.legacyPackages."${system}".pkgs;
+    pkgs = legacyPkgs;
+  in {
+
+    packages."${system}" = rec {
+
+      neovim = legacyPkgs.neovim-unwrapped.overrideAttrs(oa: {
+        version = "master";
+        src = ../.;
+
+        buildInputs = oa.buildInputs ++ ([
+          pkgs.tree-sitter
+        ]);
+
+        cmakeFlags = oa.cmakeFlags ++ [
+          "-DUSE_BUNDLED=OFF"
+        ];
+      });
+
+      # a development binary to help debug issues
+      neovim-debug = (neovim.override {
+          stdenv = pkgs.llvmPackages_latest.stdenv;
+          lua = pkgs.enableDebugging legacyPkgs.luajit;
+        }).overrideAttrs(oa:{
+          cmakeBuildType="Debug";
+          cmakeFlags = oa.cmakeFlags ++ [
+            "-DMIN_LOG_LEVEL=0"
+          ];
+      });
+
+      # for neovim developers, very slow
+      # brings development tools as well
+      neovim-developer = let
+        lib = nixpkgs.lib;
+        pythonEnv = legacyPkgs.python3;
+        luacheck = legacyPkgs.luaPackages.luacheck;
+      in
+        neovim-debug.overrideAttrs(oa: {
+          cmakeFlags = oa.cmakeFlags ++ [
+            "-DLUACHECK_PRG=${luacheck}/bin/luacheck"
+            "-DMIN_LOG_LEVEL=0"
+            "-DENABLE_LTO=OFF"
+            "-DUSE_BUNDLED=OFF"
+            # https://github.com/google/sanitizers/wiki/AddressSanitizerFlags
+            # https://clang.llvm.org/docs/AddressSanitizer.html#symbolizing-the-reports
+            "-DCLANG_ASAN_UBSAN=ON"
+          ];
+
+        nativeBuildInputs = oa.nativeBuildInputs ++ (with pkgs; [
+            pythonEnv
+            include-what-you-use  # for scripts/check-includes.py
+            jq                    # jq for scripts/vim-patch.sh -r
+            doxygen
+        ]);
+
+        shellHook = oa.shellHook + ''
+          export NVIM_PYTHON_LOG_LEVEL=DEBUG
+          export NVIM_LOG_FILE=/tmp/nvim.log
+
+          export ASAN_OPTIONS="log_path=./test.log:abort_on_error=1"
+          export UBSAN_OPTIONS=print_stacktrace=1
+        '';
+      });
+    };
+
+    defaultPackage."${system}" = self.packages."${system}".neovim;
+
+    overlay = final: prev: {
+      inherit (self.packages."${system}") neovim neovim-debug;
+    };
+
+    apps."${system}" = let
+      mkApp = pkg: {
+        type = "app";
+        program = pkg + "/bin/nvim";
+      };
+    in {
+      nvim = mkApp self.packages."${system}".neovim;
+      nvim-debug = mkApp self.packages."${system}".neovim-debug;
+    };
+
+    defaultApp."${system}" = self.apps."${system}".nvim;
+
+    devShell."${system}" = self.packages."${system}".neovim-developer;
+  };
+}