Support for declarativily ignoring `X-` keys

src/dds/library_manifest.cpp

              lm::read_accumulate("Uses", uses_strings),
              lm::read_accumulate("Links", links_strings),
              lm::read_required("Name", ret.name),
+             lm::ignore_x_keys(),
              lm::reject_unknown());
 
     extend(ret.uses, ranges::views::transform(uses_strings, lm::split_usage_string));

src/libman/parse.hpp

     }
 };
 
+class ignore_x_keys {
+public:
+    bool operator()(std::string_view, std::string_view key, std::string_view) const {
+        return key.find("X-") == 0;
+    }
+};
+
 class reject_unknown {
 public:
     int operator()(std::string_view context, std::string_view key, std::string_view) const {