Add project files.

include/cliforge/cliforge.hpp

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+#pragma once
+//
+// cliforge.hpp
+//
+// cliforge: a small header-only C++20 library for building CLIs whose
+// commands are just free functions.
+//
+//   #include <cliforge/cliforge.hpp>
+//
+//   void greet(std::string name, bool loud) {
+//       std::cout << (loud ? "HELLO, " : "Hello, ") << name << (loud ? "!!" : ".") << "\n";
+//   }
+//
+//   int main(int argc, char** argv) {
+//       cliforge::Engine cli("greeter");
+//       cli.command()
+//           .keyword("greet", "Say hello to someone")
+//           .parameter("name", "Who to greet")
+//           .flag("loud", "l", "SHOUT the greeting")
+//           .describe("Greets someone by name")
+//           .action(&greet);
+//       return cli.run(argc, argv);
+//   }
+//
+// See README.md for the full guide.
+//
+#include "command.hpp"
+#include "engine.hpp"
+#include "function_traits.hpp"
+#include "levenshtein.hpp"
+#include "slot.hpp"
+#include "value.hpp"