message: `export`: add nuget export
diff: #include "vcpkg_Input.h"
  #include "vcpkg_Util.h"
  #include "Paragraphs.h"
  +#include <regex>
  namespace vcpkg::Commands::Export
  {
  @@ -135,6 +136,49 @@ namespace vcpkg::Commands::Export
  System::println(System::Color::success, R"(Files exported at: "%s")", output.generic_string());
  + const std::string nuspec_file_content_template = R"(
  +<package>
  + <metadata>
  + <id>@NUGET_ID@</id>
  + <version>@VERSION@</version>
  + <authors>cpp-packages</authors>
  + <description>
  + Placeholder description
  + </description>
  + </metadata>
  + <files>
  + <file src="exported\**" target="" />
  + <file src="exported\.vcpkg-root" target="" />
  + <file src="scripts\buildsystems\msbuild\applocal.ps1" target="build\native\applocal.ps1" />
  + <file src="scripts\buildsystems\msbuild\vcpkg.targets" target="build\native\@NUGET_ID@.targets" />
  + <file src="scripts\buildsystems\vcpkg.cmake" target="build\native\vcpkg.cmake" />
  + </files>
  +</package>
  +)";
  +
  + const std::string nuget_id = "placeholder_id";
  + const std::string nupkg_version = "1.0.0";
  + const fs::path vcpkg_root_file = (output / ".vcpkg-root");
  +
  + fs.write_contents(vcpkg_root_file, "");
  +
  + std::string nuspec_file_content = std::regex_replace(nuspec_file_content_template, std::regex("@NUGET_ID@"), nuget_id);
  + //nuspec_file_content = std::regex_replace(nuspec_file_content, std::regex("@VCPKG_DIR@"), vcpkg_root_dir.string());
  + nuspec_file_content = std::regex_replace(nuspec_file_content, std::regex("@VERSION@"), nupkg_version);
  +
  + const fs::path nuspec_file_path = paths.root / "export.nuspec";
  + fs.write_contents(nuspec_file_path, nuspec_file_content);
  +
  + const fs::path& nuget_exe = paths.get_nuget_exe();
  +
  + const std::wstring cmd_line = Strings::wformat(LR"("%s" pack -OutputDirectory "%s" "%s" -NoDefaultExcludes)", nuget_exe.native(), paths.root.native(), nuspec_file_path.native());
  +
  +
  + const int exit_code = System::cmd_execute_clean(cmd_line);
  + Checks::check_exit(VCPKG_LINE_INFO, exit_code == 0, "Error: NuGet package creation failed");
  +
  +
  +
  Checks::exit_success(VCPKG_LINE_INFO);
  }
  }
  