feat(dap): Added debugger from cpptools

Added:
  - OpenDebugAD7 from [vscode-cpptools](https://github.com/microsoft/vscode-cpptools)

It works better with `nvim-dap-ui`. With GDB you have to wait for stdout
flush to see program output, the output is in the repl instead of the
console, etc. The VSCode debugger doesn't have such issues.

nix/neovim-overlay.nix

@@ -120,6 +120,7 @@ with final.pkgs.lib; let
     # language servers, etc.
     asm-lsp # Assembly language server
     clang-tools # C/C++ language server
+    (callPackage ./cpptools.nix {}) # C/C++ debugger from VSCode | https://github.com/microsoft/vscode-cpptools
     nodePackages.bash-language-server # Bash language server
     lua-language-server
     nil # nix LSP