Implement common external function handlers in jerry-ext (#1787)

Added `handler` module to `jerry-ext` to contain implementation of
commonly used external function handlers: `assert`, `gc`, and
`print`.

Also adapted jerry-main to use jerry-ext/handler

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

CMakeLists.txt

@@ -47,6 +47,12 @@ if(JERRY_CMDLINE OR JERRY_CMDLINE_MINIMAL)
   set(JERRY_PORT_DEFAULT_MESSAGE " (FORCED BY CMDLINE)")
 endif()
 
+if(JERRY_CMDLINE)
+  set(JERRY_EXT "ON")
+
+  set(JERRY_EXT_MESSAGE " (FORCED BY CMDLINE)")
+endif()
+
 if("${PLATFORM}" STREQUAL "DARWIN")
   set(JERRY_LIBC         "OFF")
   set(JERRY_LIBM         "OFF")