Add all the C++ sources for the library proper (no unit tests)

These are taken and adapted from the MrDocs repository, commit
88954d7f00b1d7fb8de8824e422ddc8fd7081f39.

Author: gennaroprota

include/boost/handlebars/config.hpp

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+//
+//  Copyright (c) 2023 Vinnie Falco ([email protected])
+//
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          https://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/cppalliance/handlebars
+//
+
+#ifndef BOOST_HANDLEBARS_CONFIG_HPP
+#define BOOST_HANDLEBARS_CONFIG_HPP
+
+/*
+    Platform-specific things, and stuff
+    that is dependent on the toolchain.
+*/
+
+namespace boost {
+namespace handlebars {
+
+//------------------------------------------------
+//
+// Shared Libraries
+//
+//------------------------------------------------
+
+// static linking
+#if defined(BOOST_HANDLEBARS_STATIC_LINK)
+# define BOOST_HANDLEBARS_DECL
+# define BOOST_HANDLEBARS_VISIBLE
+
+// MSVC
+#elif defined(_MSC_VER)
+# define BOOST_HANDLEBARS_SYMBOL_EXPORT __declspec(dllexport)
+# define BOOST_HANDLEBARS_SYMBOL_IMPORT __declspec(dllimport)
+# if defined(BOOST_HANDLEBARS_BUILD) // building the library
+#  define BOOST_HANDLEBARS_DECL BOOST_HANDLEBARS_SYMBOL_EXPORT
+# else
+#  define BOOST_HANDLEBARS_DECL BOOST_HANDLEBARS_SYMBOL_IMPORT
+# endif
+# define BOOST_HANDLEBARS_VISIBLE
+
+// (unknown)
+#elif defined(__GNUC__)
+# if defined(BOOST_HANDLEBARS_BUILD) // building the library
+#   define BOOST_HANDLEBARS_DECL
+# else
+#   define BOOST_HANDLEBARS_DECL __attribute__((__visibility__("default")))
+#endif
+# define BOOST_HANDLEBARS_VISIBLE __attribute__((__visibility__("default")))
+#else
+# error unknown platform for dynamic linking
+#endif
+
+//------------------------------------------------
+
+#ifndef FMT_CONSTEVAL
+# if !defined(__GNUC__) && defined(_MSC_VER)
+#  define FMT_CONSTEVAL
+# endif
+#endif
+
+#if ! defined(__x86_64__) && ! defined(_WIN64) && ! defined(__aarch64__)
+# error Handlebars requires a 64-bit architecture
+#endif
+
+#ifndef BOOST_HANDLEBARS_NO_UNIQUE_ADDRESS
+# if defined(__cpp_lib_no_unique_address)
+#  define BOOST_HANDLEBARS_NO_UNIQUE_ADDRESS [[no_unique_address]]
+# else
+#  define BOOST_HANDLEBARS_NO_UNIQUE_ADDRESS
+# endif
+#endif
+
+} // handlebars
+} // boost
+
+#endif

include/boost/handlebars/dom.hpp

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+//
+// Copyright (c) 2023 Vinnie Falco ([email protected])
+//
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          https://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/cppalliance/handlebars
+//
+
+#ifndef BOOST_HANDLEBARS_DOM_HPP
+#define BOOST_HANDLEBARS_DOM_HPP
+
+#include <boost/handlebars/dom/value.hpp>
+
+#endif