Add support for nix flakes (#1614)

* Add support for nix flakes
* Added debug build type for flake.nix; Got rid of redundant version check in nix/default.nix
* Added dev shell with compile_commands.json into flake.nix
* Fixed issue with generated compile_commands.json while creating c3c derivation with nix. Deduced devShells in flake.nix to nix/shell.nix

nix/shell.nix

@@ -0,0 +1,20 @@
+{
+  mkShell,
+  clang-tools,
+  c3c,
+}:
+
+mkShell {
+  inputsFrom = [
+    c3c
+  ];
+
+  packages = [ 
+    clang-tools 
+    c3c
+  ];
+
+  shellHook = ''
+    ln -sf ${c3c}/compile_commands.json compile_commands.json
+  '';
+}