Provide support for native modules with ES6 imports (#3090)

JerryScript-DCO-1.0-Signed-off-by: Dániel Bátyai dbatyai@inf.u-szeged.hu

docs/05.PORT-API.md

@@ -83,10 +83,14 @@ information.
 void jerry_port_print_char (char c);
 ```
 
-### ES2015 Module system helper functions
+### ES2015 Module system
 
-The module system requires two specific functions for opening and closing files.
-It also requires a platform specific way of normalizing file paths.
+The port API provides functions that can be used by the module system to open
+and close source files, and normalize file paths.
+The `jerry_port_get_native_module` port function can be used to provide native
+modules to the engine. This function will be called when an import/export
+statement is encountered with an unknown module specifier, which embedders can
+use to supply native module objects based on the module name argument.
 
 ```c
 /**
@@ -126,6 +130,25 @@ jerry_port_normalize_path (const char *in_path_p, /**< input file path */
   // write to out_buf_p the normalized path
   // return length of written path
 } /* jerry_port_normalize_path */
+
+/**
+ * Get the module object of a native module.
+ *
+ * Note:
+ *      This port function is called by jerry-core when ES2015_MODULE_SYSTEM
+ *      is enabled.
+ *
+ * @param name String value of the module specifier.
+ *
+ * @return Undefined, if 'name' is not a native module
+ *         jerry_value_t containing the module object, otherwise
+ */
+jerry_value_t
+jerry_port_get_native_module (jerry_value_t name) /**< module specifier */
+{
+  (void) name;
+  return jerry_create_undefined ();
+}
 ```
 
 ## Date

docs/15.MODULE-SYSTEM.md

@@ -2,6 +2,7 @@
 
 The module system allows users to write import and export statements in scripts, which can be used to separate the logic of the application into custom modules.
 The standard's relevant part can be found [here](https://www.ecma-international.org/ecma-262/6.0/#sec-modules).
+Embedders wishing to use native builtin modules with ES6 imports can use the [Port API](05.PORT-API.md#es2015-module-system) to do so.
 
 ## General