Implement a is_known_schema convenience function (#2280)

Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

src/core/jsonschema/bundle.cc

@@ -19,7 +19,7 @@ auto is_official_metaschema_reference(
   assert(!pointer.empty());
   assert(pointer.back().is_property());
   return pointer.back().to_property() == "$schema" &&
-         sourcemeta::core::schema_resolver(destination).has_value();
+         sourcemeta::core::is_known_schema(destination);
 }
 
 auto dependencies_internal(const sourcemeta::core::JSON &schema,

src/core/jsonschema/include/sourcemeta/core/jsonschema.h

@@ -40,6 +40,11 @@ namespace sourcemeta::core {
 SOURCEMETA_CORE_JSONSCHEMA_EXPORT
 auto schema_resolver(const std::string_view identifier) -> std::optional<JSON>;
 
+/// @ingroup jsonschema
+/// Check if a given identifier corresponds to a known built-in schema
+SOURCEMETA_CORE_JSONSCHEMA_EXPORT
+auto is_known_schema(const std::string_view identifier) noexcept -> bool;
+
 /// @ingroup jsonschema
 /// A default schema walker with support for a wide range of drafts
 SOURCEMETA_CORE_JSONSCHEMA_EXPORT